law of reflection assignment

• Reflection Of Light

Reflection of Light

Have you ever thought about why we can see our image in a plane mirror? It’s because of the phenomenon known as reflection. Light waves, sound waves, and water waves can undergo reflection. In this session, let us learn about the reflection of light and the types of reflection in detail.

What Is Reflection of Light?

When a ray of light approaches a smooth polished surface and the light ray bounces back, it is called the reflection of light. The incident light ray that land on the surface is reflected off the surface. The ray that bounces back is called the reflected ray. If a perpendicular were drawn on a reflecting surface, it would be called normal. The figure below shows the reflection of an incident beam on a plane mirror.

Here, the angle of incidence and angle of reflection are with respect to normal and the reflective surface.

Laws of Reflection

The laws of reflection determine the reflection of incident light rays on reflecting surfaces, like mirrors, smooth metal surfaces and clear water. Let’s consider a plane mirror as shown in the figure above. The law of reflection states that

• The incident ray, the reflected ray and the normal all lie in the same plane
• The angle of incidence = Angle of reflection

Watch the video and learn more about laws of reflection

Types of Reflection of Light

Different types of reflection of light are briefly discussed below:

• Regular reflection is also known as specular reflection
• Diffused reflection
• Multiple reflection

Regular/ Specular Reflection

Specular Reflection refers to a clear and sharp reflection, like the ones you get in a mirror. A mirror is made of glass coated with a uniform layer of a highly reflective material such as powder. This reflective surface reflects almost all the light incident on it uniformly. There is not much variation in the angles of reflections between various points. This means that the haziness and the blurring are almost entirely eliminated.

Regular Specular Reflection

Diffused Reflection

Reflective surfaces other than mirrors, in general, have a very rough finish. This may be due to wear and tear such as scratches and dents or dirt on the surface. Sometimes even the material of which the surface is made of matters. All this leads to a loss of both the brightness and the quality of the reflection.

In the case of such rough surfaces, the angle of reflection when compared between points is completely haphazard. For rough surfaces, the rays incident at slightly different points on the surface is reflected in completely different directions. This type of reflection is called diffused reflection and is what enables us to see non-shiny objects.

Multiple Reflection

A single image is formed when an object is placed in front of a mirror. What happens if we use two mirrors? Since reflective surfaces such as mirrors are very good at preserving the intensity of light in a reflection, a single light source can be reflected multiple times. These multiple reflections are possible until the intensity of light becomes low to the point that we cannot see. This means that we can have almost infinite multiple reflections. We can also see an image in every individual reflection. This means that each image is the result of an image or an image of an image.

The number of images we see depends on the angle between the two mirrors. We see that as we go on decreasing the angle between the mirrors, the number of images increases. And when the angle becomes zero, i.e., when the mirrors become parallel, the number of images becomes infinite. This effect can be easily observed when your barber uses another smaller mirror to show you the back of your head. When this happens, not only do you see the back of your head, but you also see innumerable images of yourself. The variation of the number of images of an object placed between two mirrors with the angle between the mirrors can be described by a simple formula:

\(\begin{array}{l}Number\; of \; images = \frac{360^{\circ}}{angle\; between\; mirrors}-1\end{array} \)

Frequently Asked Questions – FAQs

What is meant by reflection of light.

When a light ray approaches a smooth polished surface and the light ray bounces back, it is known as the reflection of light.

What is interference?

Interference is the phenomenon in which two waves superpose to form the resultant wave of the lower, higher or same amplitude.

State the laws of reflection?

• The incident ray, the reflected ray and the normal all lie in the same plane.
• The angle of incidence is equal to the angle of reflection.

What are the types of reflection of light?

• Regular reflection/specular reflection

Which type of reflection results in a clear and sharp reflection?

Recommended videos, light top 20 question and answers class 10.

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Examples of Reflective Writing

Types of reflective writing assignments.

A journal  requires you to write weekly entries throughout a semester. May require you to base your reflection on course content.

A learning diary is similar to a journal, but may require group participation. The diary then becomes a place for you to communicate in writing with other group members.

A logbook is often used in disciplines based on experimental work, such as science. You note down or 'log' what you have done. A log gives you an accurate record of a process and helps you reflect on past actions and make better decisions for future actions.

A reflective note is often used in law. A reflective note encourages you to think about your personal reaction to a legal issue raised in a course.

An essay diary  can take the form of an annotated bibliography (where you examine sources of evidence you might include in your essay) and a critique (where you reflect on your own writing and research processes).

a peer review  usually involves students showing their work to their peers for feedback.

A self-assessment task  requires you to comment on your own work.

Some examples of reflective writing

Social science fieldwork report (methods section), engineering design report, learning journal (weekly reflection).

Brookfield, S 1987, Developing critical thinkers: challenging adults to explore alternative ways of thinking and acting , Open University Press, Milton Keynes.

Mezirow, J 1990, Fostering critical reflection in adulthood: a guide to transformative and emancipatory learning , Jossey-Bass, San Francisco.

Schön, DA 1987, Educating the reflective practitioner , Jossey-Bass. San Francisco.

We thank the students who permitted us to feature examples of their writing.

Prepared by Academic Skills, UNSW. This guide may be distributed or adapted for educational purposes. Full and proper acknowledgement is required. 

Essay and assignment writing guide

• Essay writing basics
• Essay and assignment planning
• Answering assignment questions
• Editing checklist
• Writing a critical review
• Annotated bibliography
• How do I write reflectively?
• Examples of reflective writing
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1.2: Reflection at a Plane Surface

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• Jeremy Tatum
• University of Victoria

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The law of reflection of light is merely that the angle of reflection \(r\) is equal to the angle of incidence \(i\). There is really very little that can be said about this, but I’ll try and say what little need be said.

• It is customary to measure the angles of incidence and reflection from the normal to the reflecting surface rather than from the surface itself.
• Some curmudgeonly professors may ask for the law S of reflection, and will give you only half marks if you neglect to add that the incident ray, the reflected ray and the normal are coplanar.
• A plane mirror forms a virtual image of a real object :

or a real image of a virtual object :

• It is usually said that the image is as far behind the mirror as the object is in front of it. In the case of a virtual object (i.e. light converging on the mirror, presumably from some large lens somewhere to the left) you’d have to say that the image is as far in front of the mirror as the object is behind it!
• If the mirror were to move at speed \(v\) away from a real object, the virtual image would move at speed \(2v\). I’ll leave you to think about what happens in the case of a virtual object.
• If the mirror were to rotate through an angle \( \theta \) (or were to rotate at an angular speed ω), the reflected ray would rotate through an angle \( 2 \theta \) (or at an angular speed \( 2 \omega\)).
• Only smooth, shiny surfaces reflect light as described above. Most surfaces, such as paper, have minute irregularities on them, which results in light being scattered in many directions. Various equations have been proposed to describe this sort of scattering. If the reflecting surface looks equally bright when viewed from all directions, the surface is said to be a perfectly diffusing Lambert’s law surface. Reflection according to the \(r = i \) law of reflection, with the incident ray, the reflected ray and the normal being coplanar, is called specular reflection (Latin: speculum, a mirror). Most surfaces are intermediate between specular reflectors and perfectly diffusing surfaces. This chapter deals exclusively with specular reflection.
• The image in a mirror is reversed from left to right, and from back to front, but is not reversed up and down. Discuss.
• If you haven’t read Through the Looking-glass and What Alice Found There , you are missing something.

Light goes from A to B via reflection from a point P in a mirror.

The distance \(s\) traveled is given by

\[ s = \sqrt{a^2 + x^2} + \sqrt{a^2 + ( b - x )^2 } \label{eq:1.2.1} \]

Here is the distance traveled as a function of the position of the point P:

The path that the light actually takes is the path such that the distance traveled is a minimum, which is such that P is horizontally halfway between A and B. You can see this from the graph, or by differentiating the above expression for \(s\). This means that the angle of reflection is equal to the angle of incidence. You may regard this observation as a slightly interesting trivium, or as a fundamental principle of the deepest significance. Whichever you choose, you will come across lots of other examples of nature operating with Least Action. And you won’t have to wait long. There’s another one in the next section.

25.3 The Law of Refraction

Learning objectives.

By the end of this section, you will be able to:

• Determine the index of refraction, given the speed of light in a medium.

It is easy to notice some odd things when looking into a fish tank. For example, you may see the same fish appearing to be in two different places. (See Figure 25.10 .) This is because light coming from the fish to us changes direction when it leaves the tank, and in this case, it can travel two different paths to get to our eyes. The changing of a light ray’s direction (loosely called bending) when it passes through variations in matter is called refraction . Refraction is responsible for a tremendous range of optical phenomena, from the action of lenses to voice transmission through optical fibers.

The changing of a light ray’s direction (loosely called bending) when it passes through variations in matter is called refraction.

Speed of Light

The speed of light c c not only affects refraction, it is one of the central concepts of Einstein’s theory of relativity. As the accuracy of the measurements of the speed of light were improved, c c was found not to depend on the velocity of the source or the observer. However, the speed of light does vary in a precise manner with the material it traverses. These facts have far-reaching implications, as we will see in Special Relativity . It makes connections between space and time and alters our expectations that all observers measure the same time for the same event, for example. The speed of light is so important that its value in a vacuum is one of the most fundamental constants in nature as well as being one of the four fundamental SI units.

Why does light change direction when passing from one material (medium) to another? It is because light changes speed when going from one material to another. So before we study the law of refraction, it is useful to discuss the speed of light and how it varies in different media.

The Speed of Light

Early attempts to measure the speed of light, such as those made by Galileo, determined that light moved extremely fast, perhaps instantaneously. The first real evidence that light traveled at a finite speed came from the Danish astronomer Ole Roemer in the late 17th century. Roemer had noted that the average orbital period of one of Jupiter’s moons, as measured from Earth, varied depending on whether Earth was moving toward or away from Jupiter. He correctly concluded that the apparent change in period was due to the change in distance between Earth and Jupiter and the time it took light to travel this distance. From his 1676 data, a value of the speed of light was calculated to be 2 . 26 × 10 8 m/s 2 . 26 × 10 8 m/s (only 25% different from today’s accepted value). In more recent times, physicists have measured the speed of light in numerous ways and with increasing accuracy. One particularly direct method, used in 1887 by the American physicist Albert Michelson (1852–1931), is illustrated in Figure 25.11 . Light reflected from a rotating set of mirrors was reflected from a stationary mirror 35 km away and returned to the rotating mirrors. The time for the light to travel can be determined by how fast the mirrors must rotate for the light to be returned to the observer’s eye.

The speed of light is now known to great precision. In fact, the speed of light in a vacuum c c is so important that it is accepted as one of the basic physical quantities and has the fixed value

where the approximate value of 3 . 00 × 10 8 m/s 3 . 00 × 10 8 m/s is used whenever three-digit accuracy is sufficient. The speed of light through matter is less than it is in a vacuum, because light interacts with atoms in a material. The speed of light depends strongly on the type of material, since its interaction with different atoms, crystal lattices, and other substructures varies. We define the index of refraction n n of a material to be

where v v is the observed speed of light in the material. Since the speed of light is always less than c c in matter and equals c c only in a vacuum, the index of refraction is always greater than or equal to one.

Value of the Speed of Light

Index of refraction.

That is, n ≥ 1 n ≥ 1 . Table 25.1 gives the indices of refraction for some representative substances. The values are listed for a particular wavelength of light, because they vary slightly with wavelength. (This can have important effects, such as colors produced by a prism.) Note that for gases, n n is close to 1.0. This seems reasonable, since atoms in gases are widely separated and light travels at c c in the vacuum between atoms. It is common to take n = 1 n = 1 for gases unless great precision is needed. Although the speed of light v v in a medium varies considerably from its value c c in a vacuum, it is still a large speed.

Example 25.1

Speed of light in matter.

Calculate the speed of light in zircon, a material used in jewelry to imitate diamond.

The speed of light in a material, v v , can be calculated from the index of refraction n n of the material using the equation n = c / v n = c / v .

The equation for index of refraction states that n = c / v n = c / v . Rearranging this to determine v v gives

The index of refraction for zircon is given as 1.923 in Table 25.1 , and c c is given in the equation for speed of light. Entering these values in the last expression gives

This speed is slightly larger than half the speed of light in a vacuum and is still high compared with speeds we normally experience. The only substance listed in Table 25.1 that has a greater index of refraction than zircon is diamond. We shall see later that the large index of refraction for zircon makes it sparkle more than glass, but less than diamond.

Law of Refraction

Figure 25.12 shows how a ray of light changes direction when it passes from one medium to another. As before, the angles are measured relative to a perpendicular to the surface at the point where the light ray crosses it. (Some of the incident light will be reflected from the surface, but for now we will concentrate on the light that is transmitted.) The change in direction of the light ray depends on how the speed of light changes. The change in the speed of light is related to the indices of refraction of the media involved. In the situations shown in Figure 25.12 , medium 2 has a greater index of refraction than medium 1. This means that the speed of light is less in medium 2 than in medium 1. Note that as shown in Figure 25.12 (a), the direction of the ray moves closer to the perpendicular when it slows down. Conversely, as shown in Figure 25.12 (b), the direction of the ray moves away from the perpendicular when it speeds up. The path is exactly reversible. In both cases, you can imagine what happens by thinking about pushing a lawn mower from a footpath onto grass, and vice versa. Going from the footpath to grass, the front wheels are slowed and pulled to the side as shown. This is the same change in direction as for light when it goes from a fast medium to a slow one. When going from the grass to the footpath, the front wheels can move faster and the mower changes direction as shown. This, too, is the same change in direction as for light going from slow to fast.

The amount that a light ray changes its direction depends both on the incident angle and the amount that the speed changes. For a ray at a given incident angle, a large change in speed causes a large change in direction, and thus a large change in angle. The exact mathematical relationship is the law of refraction , or “Snell’s Law,” which is stated in equation form as

Here n 1 n 1 and n 2 n 2 are the indices of refraction for medium 1 and 2, and θ 1 θ 1 and θ 2 θ 2 are the angles between the rays and the perpendicular in medium 1 and 2, as shown in Figure 25.12 . The incoming ray is called the incident ray and the outgoing ray the refracted ray, and the associated angles the incident angle and the refracted angle. The law of refraction is also called Snell’s law after the Dutch mathematician Willebrord Snell (1591–1626). While the law has been named after Snell, the Arabian physicist, Ibn Sahl, found the law of refraction in 984 and used it in his work On Burning Mirrors and Lenses. Snell’s experiments showed that the law of refraction was obeyed and that a characteristic index of refraction n n could be assigned to a given medium. Snell was not aware that the speed of light varied in different media, but through experiments he was able to determine indices of refraction from the way light rays changed direction.

The Law of Refraction

Take-home experiment: a broken pencil.

A classic observation of refraction occurs when a pencil is placed in a glass half filled with water. Do this and observe the shape of the pencil when you look at the pencil sideways, that is, through air, glass, water. Explain your observations. Draw ray diagrams for the situation.

Example 25.2

Determine the index of refraction from refraction data.

Find the index of refraction for medium 2 in Figure 25.12 (a), assuming medium 1 is air and given the incident angle is 30 . 0º 30 . 0º and the angle of refraction is 22 . 0º 22 . 0º .

The index of refraction for air is taken to be 1 in most cases (and up to four significant figures, it is 1.000). Thus n 1 = 1 . 00 n 1 = 1 . 00 here. From the given information, θ 1 = 30 . 0º θ 1 = 30 . 0º and θ 2 = 22 . 0º θ 2 = 22 . 0º . With this information, the only unknown in Snell’s law is n 2 n 2 , so that it can be used to find this unknown.

Snell’s law is

Rearranging to isolate n 2 n 2 gives

Entering known values,

This is the index of refraction for water, and Snell could have determined it by measuring the angles and performing this calculation. He would then have found 1.33 to be the appropriate index of refraction for water in all other situations, such as when a ray passes from water to glass. Today we can verify that the index of refraction is related to the speed of light in a medium by measuring that speed directly.

Example 25.3

A larger change in direction.

Suppose that in a situation like that in Example 25.2 , light goes from air to diamond and that the incident angle is 30 . 0º 30 . 0º . Calculate the angle of refraction θ 2 θ 2 in the diamond.

Again the index of refraction for air is taken to be n 1 = 1 . 00 n 1 = 1 . 00 , and we are given θ 1 = 30 . 0º θ 1 = 30 . 0º . We can look up the index of refraction for diamond in Table 25.1 , finding n 2 = 2 . 419 n 2 = 2 . 419 . The only unknown in Snell’s law is θ 2 θ 2 , which we wish to determine.

Solving Snell’s law for sin θ 2 θ 2 yields

The angle is thus

For the same 30º 30º angle of incidence, the angle of refraction in diamond is significantly smaller than in water ( 11.9º 11.9º rather than 22º 22º —see the preceding example). This means there is a larger change in direction in diamond. The cause of a large change in direction is a large change in the index of refraction (or speed). In general, the larger the change in speed, the greater the effect on the direction of the ray.

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Introducing reflection as an assignment

Using reflective assignments can be a great way of synthesising learning and challenging the status quo. This page outlines some of the things to keep in mind when posing reflective assignments.

In higher education or professional develop initiatives it is very common to have some sort of assignment. These are typically written but can also take other forms. This page will go through the main considerations for posing reflective assignments.

The main points covered are:

• finding and communicating the purpose of your assignment
• being clear both to yourself and to reflector what you want in the assignment
• the difference between ‘reflection’ and ‘evidence of reflection’
• choosing your criteria
• providing students support and spending time practicing can be valuable as most students are new to reflection.

Back to alignment – find the purpose of the assignment and communicate it

It should be clear to participants or students what the purpose of the assignment is. Why are you asking them to do this particular assignment? You will have had to think about the value of it.

This value can be described in the guidelines of the reflective assignment where you communicate how it will help reflectors either evidence their learning or obtain learning outcomes. From the guidelines it should be clear to students what the value of completing and doing well on the assignment is.

Be clear what you are asking

When posing a reflective assignment it is very important that you know from the beginning exactly what you are asking. Reflective writing/responses can typically take on two distinct forms:

• reflection,
• evidence of reflection.

The distinction between the two is vital when deciding the type of assignment you want to pose. These are outlined below.

Reflection - the actual process of examining thoughts

If you want to see the detailed aspects of reflectors’ thought processes, and want to follow each step in their reasoning, concerns, and learnings, ask the reflectors to submit their actual reflections.

The benefits is that you ensure that reflectors go through the process themselves and you can directly assess the quality. As this is the actual process we want the reflectors to complete, asking for raw reflections is the easiest way to ensure or get evidence that the process is happening.

One challenge when posing this kind of assignment is that some people might find it too personal to share this intimate process – it can become self-disclosure. A personal reflective account can be uncomfortable to show to anyone, and even more so to someone who is in a position of authority.

Evidence of reflection

In contrast, ‘evidence of reflection’ is documenting the effects of reflection, but does not require documenting the process explicitly.

Hence, rather than writing the thoughts and feelings of a situation, the reflector will state the context and what learning they found in the experience. In the purest form, there is no need to document any challenging or self-disclosing feelings. It is more akin to describing the effects of a reflection and rationally, in contrast to emotionally, explaining why the learning is valuable.

The benefit of this is that reflectors are less likely to feel that they are self-disclosing. However, when we are looking at evidence of reflection rather than reflection itself, it is more difficult to assess the reflectors ability to actually reflect. Therefore, good evidence of reflection is when learning is explicitly stated and it is highlighted how the learning will be used in the future.

It is important to be aware that there is a risk, albeit minimal, that a reflector can produce good evidence of reflection, without having done any reflection. For example, a reflector may write that they learned to start assignments earlier and will do so in the future, without actually having engaged with reflection at all – they might just guess that ‘starting assignments earlier’ is a possible conclusion you want to see.

Most assignments are a balance of ‘reflection’ and ‘evidence of reflection’

In reality, very few assignments will be a either pure ‘reflection’ or ‘evidence of reflection’. The goal for you is to find the right balance. Once you know what you want, you should be clear to reflectors about what being successful in the assignment looks like.

The easiest way to demonstrate what good looks like is to provide the reflectors with clear guidelines and examples of the type of reflections you are looking for. You can either write examples yourself or have a look through the Reflectors’ Toolkit, where each of the models have at least one example. You will likely find an example there that can be helpful for you.

List of tools for reflection (in Reflectors’ Toolkit) (LINK)

Reflection is just like any other assignment – avoid vagueness

The need for clear assignment directions is essential in all areas of higher education, however having the discussion specifically for reflection is important. This is because when posing a reflective assignment it can feel easy to consider reflection as ‘special’ and separate from common ‘good academic practice’ and therefore that it does not require the same levels of direction as a general assignment. Reflection should be considered on equal terms with general academic practice and will often require more support as many reflectors are new to the concept.

One reason vague reflection assignments are easy to pose is that they do not seem to restrict the reflectors’ freedom about how to reflect. In contrast, if we provide them with clear requirements and directions it might seem that we do restrict reflection. There is an element of truth in that. If we require as written assignment using a specific model of reflection, we do take some freedom away from the reflectors, at least in how they present their reflections to us. In practice, they can easily produce a private reflection and restructure it according to your question and requirements.

If we do not give the reflectors the structure they need, one challenge is that a high proportion of them might produce reflections not meeting our ideas of sufficient or good.

Posing a reflective assignment saying ‘Reflect on your development and learning in the course in 1000 words’ might seem like a fair question to ask. But compare that to asking them to ‘write an academic essay about the concepts you learned in this course in 1000 words’ and it should be clear why guidelines are important. It is easy to imagine how students would struggle to prioritise and produce an essay with relevant content from the vague essay prompt. This is similar for a vaguely posed reflective assignment without accompanying clear guidelines. How are the reflectors going to guess what we expect from them?

Most people are new to structured reflection

In higher education, most people have an idea of what an essay is supposed to look like because we are taught essay writing from an early age in school. In contrast, most people have never done structured reflection before university, and then are not likely to be thoroughly instructed in how to do or present it. It follows that if we are vague in our instructions we may receive assignments of very varying qualities.

Thus, to be fair to the reflectors and to us as facilitators, be clear and have clear guidelines available. You can ask very broad reflective questions, but you should be ready to support the reflectors and both your criteria and rubrics (if you chose to assess) should be extremely robust.

Providing training/introductions to students is useful

As most people are new to reflection starting in university, when you introduce reflection it can helpful to: provide a thorough written guide of what reflection is, provide people with resources (for example the Reflectors’ Toolkit), and/or spend time in person introducing reflectors to structured reflection and what you expect from reflections.

Find your criteria and your rubric

Once you have a clear assignment, it is important you think about what you want to measure it against, i.e. the criteria. This discussion is also highlighted in the ‘Assessing reflection’ section of the Facilitators’ Toolkit with specific criteria as suggestions.

Moreover, if you decide to use summative assessment for the assignments, you will need to have a clear rubric (criteria broken down into levels of performance). It is good practice to publish both the criteria and rubric to the reflectors prior to assessing them.

To see at what point criteria and rubrics become essential, see ‘Should I assess?’

Assessing reflection (within the Facilitators’ Toolkit)

Should I assess? (within the Facilitators’ Toolkit)

Back to 'How do I introduce reflection?'

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Fundamentals

A light ray reflects (bounces off) a plane mirror surface in a very predictable way. The angle at which it approaches the mirror is equal to the angle at which it reflects from the mirror. This is known as the  law of reflection .  

About This Question

There are three similar versions of this question. Here is one of those versions:  

The diagram shows an incident ray approaching a mirror surface. Tap on the diagram in order to identify the appropriate reflected ray.       

How to Think About This Situation:

About the Diagram The diagram shows a plane mirror represented by the bold, thick angled line. The diagram also shows an incident ray - ray approaching the mirror - represented by the red arrow. The mirror and incident ray are placed on top of a  protractor  with angle divisions every 15 degrees. The protractor will be one of the most important tools in answering this question. Applying the Law of Reflection The law of reflection can be simply stated as ...  

Learn More at The Physics Classroom Tutorial

To learn more about the law of reflection for plane mirrors, visit the following page at The Physics Classroom Tutorial: The Law of Reflection