Light (VIVA EDUCATION)
EXERCISES
BE PROMPT
A. Fill in the blanks.
1. The image that can be obtained on a screen is called real image.
2. Unlike a pinhole camera, a plane mirror forms erect image.
3. We see the objects around us because they reflect light.
4. A highly polished glass table will show regular reflection.
5. When a ray of light falls normally on a plane mirror, the angle of reflection is 0°.
6. The angle between the incident ray and the mirror is equal to the angle between the reflected ray and the mirror.
7. The speed of light slows down on entering a medium from vacuum.
B. State whether the following statements are true (T) or false (F). Correct the false statements.
1. Plane mirror is a good reflector. [T]
2. The laws of reflection are not valid when the incident and the reflected rays are reversed. [F]
= The laws of reflection are also valid when the incident and the reflected rays are reversed.
3. In case of irregular reflection, no definite image is formed. [T]
4. Plane mirrors form a real and inverted image. [F]
= Plane mirrors form a virtual and erect image.
5. White light is obtained on mixing the secondary colours. [F]
= White light is obtained on mixing the primary colours.
6. Light always needs a medium to pass. [F]
= Light does not need a medium to pass.
7. An object which absorbs lights of all colours appears dark. [T]
C. Choose the correct option.
1. Which of the following statements is not true about the reflection of light?
(a) All substances absorb some incident light and reflect the rest. [ ]
(b) Highly polished surfaces are good reflectors of light. [ ]
(c) The amount of light reflected from a surface depends on the nature of the surface. [ ]
(d) An opaque object absorbs most of the incident light and reflects only a little. [✓]
2. According to the laws of reflection, the angle of incidence is _________ the angle of reflection.
(a) less than. [ ]
(b) equal to. [✓]
(c) greater than. [ ]
(d) always twice. [ ]
3. The normal to the surface is drawn __________ to the surface.
(a) parallel. [ ]
(b) at an angle. [ ]
(c) perpendicular. [✓]
(d) none of these. [ ]
4. For a plane mirror, the distance between the object and the mirror is always _________ distance between the image and the mirror.
(a) less than. [ ]
(b) equal to. [✓]
(c) greater than. [ ]
(d) half. [ ]
5. Which of the following is/are example(s) of diffused reflection?
(a) Reflection in a plane mirror. [ ]
(b) Reflection in a scratched mirror. [✓]
(c) Reflection in an unused steel plate. [ ]
(d) All of them. [ ]
6. Which of the following is not a characteristic of the image formed by a plane mirror?
(a) Inverted. [✓]
(b) Virtual. [ ]
(c) Formed behind the mirror. [ ]
(d) Laterally Inverted. [ ]
D. Match the columns.
=
|
Column A |
Column B |
|---|---|
|
1. Yellow. |
(a) Dispersion of light. [3] |
|
2. Blue. |
(b) Specular reflection. [4] |
|
3. Rainbow. |
(c) Primary colour. [2] |
|
4. Still water. |
(d) Diffused reflection. [5] |
|
5. Rippled water. |
(e) Secondary colour. [1] |
SHORT AND PRECISE
A. Show the following in the form of flowcharts.
1. Types of reflections with their diagrams.
=
2. Types of colours.
=
B. Give reasons for the following statements.
1. We cannot see things kept in dark.
= We can only able to see objects around us if light reflected by them enters our eyes. That is why we cannot see things kept in dark.
2. We see our erect image in the mirror at home.
= Because rays of light reflect on the mirror such a way that they appear to meet at a certain point but do not actually meet. Mirror forms virtual image which are erect in nature.
3. A light ray incident normally on a mirror retraces its path.
= Because a mirror causes regular reflection in which the angle of incidence is equal to the angle of reflection. That is why a light ray incident normally on a mirror retraces its path in the perpendicular way.
4. When we raise our left hand in mirror, it appears that our right hand is raised.
= Because lateral inversion happens in the image formed by a mirror.
5. If red rose is seen in green light, it appears darker.
= If red rose is seen in green light, it absorbs the green colour and reflects nothing. Therefore, it appears darker.
C. Answer in short.
1. What is reflection of light? How does it help us?
= The bouncing back of light from the surface of an object in the same medium known as reflection of light.
We are able to see objects around us with the light reflected by them enters our eyes.
2. Explain a mirror and its classification.
= A mirror is a smooth and highly polished surface that reflects all the light falling on it.
The smooth surface can be flat or a little curved. On this basis, we classify mirrors as plane and spherical.
Plane mirror have a flat reflecting surface.
Spherical mirrors are further classified as concave mirrors and convex mirrors. The reflecting surface of a concave mirror is curved inwards whereas the reflecting surface of a convex mirror is bulged outwards.
3. What happens when the incident ray strikes the reflecting surface normally?
= When the incident ray strikes the reflecting surface normally, the reflected ray retraces the path in backward direction. In this case, ã„¥i=ã„¥r=0°.
4. What are the factors on which the reflection of light from any surface depends?
= Reflection of light depends upon the nature of object on which it falls. A transparent substance allows the light to pass through and does not reflect any light which is why it appears transparent. An opaque substance absorbs a little but reflects most of the light that falls on it. A smooth and highly polished surface such as a mirror reflects all the light that falls on it.
5. Draw a well-labelled diagram to explain image formation of a large object by a plane mirror.
= Suppose a large object AB is placed in front of a plane mirror XY. It reflects light rays in all the directions. We only consider the rays of light starting from A and B. They fall on the surface of the mirror and are reflected in a particular direction obeying the laws of reflection.
The reflected beam form A, after suffering reflection, when reaches our eyes appears to be coming from A'. Similarly, the rays from B appears to be coming from B'. Thus the image is formed at A'B' behind the mirror.
6. Find out letters in English alphabets which resemble their image formed by a plane mirror.
= Letters in English alphabets which resemble their image formed by a plane mirror are - A, H,I,M,O,T,U,V,W,X and Y.
7. What is a virtual image? Give one situation in which virtual image is formed.
= When the rays of light are reflected in such a way that they appear to meet at a certain point but do not actually meet on the screen, the image so formed is called virtual image.
Virtual image is formed in the plane mirror.
8. Rohan is observing his image in a plane mirror. The distance between the mirror and his image is 4m. If he moves 1m towards the mirror, find the distance between the image and the mirror?
= Mirror to Rohan = 4m.
Now, Mirror to Rohan = 4m - 1m = 3m.
The distance between the image and the mirror is the same as that of the object and the mirror.
So, the distance between image and the mirror = 3m+3m = 6m.
AT LENGTH
A. Explain the following terms.
1. Reflecting surface.
= The smooth shiny surface which reflects the light.
2. Lateral inversion.
= The phenomenon in which the left side of the object appears to be right side of the image formed by the plane mirror and vice versa.
3. Normal.
= The perpendicular drawn at the point of the incidence on the reflecting surface in the same medium.
4. Angle of incidence.
= The angle which the incident ray makes with the normal.
5. Angle of reflection.
= The angle which the reflected ray makes with the normal.
6. White light.
= On mixing the primary colours, the get white light.
B. Differentiate between the following.
1. Regular and irregular reflection with examples.
=
|
Regular Reflection |
Irregular Reflection |
|---|---|
|
1.It happens when light falls on highly polished
smooth surface. |
1. It happens when light falls on rough surface. |
|
2. All the rays of the beam are reflected parallel
to each other. |
2. All the rays of the beam are reflected in
different direction. |
|
3. Example – Plane Mirror. |
3. Example – road, door, ordinary floor and table. |
2. Reflection and dispersion of light.
=
|
Reflection |
Dispersion |
|---|---|
|
1. The bouncing back of light from the surface of an
object in the same medium is known as reflection of light. |
2. The phenomenon of splitting of white light into
several component colours is called dispersion of light. |
|
2. Light need not to pass through a medium. |
2. Light needs to pass through a denser medium. |
|
3. The colour of incident ray and reflected ray is
the same. |
4. The colour of incident ray and dispersed rays are
different. |
3. Real and virtual image.
=
|
Real Image |
Virtual Image |
|---|---|
|
1. It can be taken on a screen. |
1. It cannot be taken on a screen. |
|
2. It is always inverted. |
2. It is always erect. |
|
3. It is formed in front of the mirror. |
3. It is formed behind the mirror. |
4. Primary and secondary colours.
=
|
Primary Colour |
Secondary Colour |
|---|---|
|
1. These colours cannot be obtained or created by mixing
other colours. |
1. These colours can be produced by mixing two
primary colours. |
|
2. Red, Green and Blue are the primary colours. |
2. Yellow, Cyan and Magenta are the secondary colours. |
|
3. They can produce white colour by mixing all
together. |
3. They can not produce white light anyway. |
C. Read the given process carefully. Identify the incorrect parts of process.
When a parallel beam of light falls on a highly polished and smooth surface such that all the rays of the beam are reflected divergent to each other, then the dispersion is known as irregular or specular reflection. Reflection in a plane mirror, rippling water and highly polished floor are the examples of irregular reflection.
= When a parallel beam of light falls on a highly polished and smooth surface such that all the rays of the beam are reflected parallel to each other, then the dispersion is known as regular or specular reflection. Reflection in a plane mirror, rippling water and highly polished floor are the examples of regular reflection.
D. Answer in detail.
1. With the help of a labelled diagram, explain all the terms related to reflection.
= Terms related to reflection :
❐ Reflecting surface : The smooth shiny surface which reflects the light is known as reflecting surface. In the diagram, XY is the reflecting surface.
❐ Incident ray : The ray of light which is falling on the reflecting surface XY is called the incident ray. AO is the incident ray.
❐ Reflected ray : The ray of light which bounces back after striking the reflecting surface XY in the same medium is called the reflected ray. OB is the reflected ray.
❐ Point of incidence : The point where the incident ray strikes on the reflecting surface XY is known as the point of incidence. O is the point of incidence. It is the same point from where the reflected ray originates.
❐ Normal : It is the perpendicular drawn on the reflecting surface at the point of incidence. ON is the normal to the reflecting surface.
❐ Angle of incidence : The angle which the incident ray makes with the normal is know as the angle of incidence. It is denoted by i. ∠AON is the angle of incidence.
❐ Angle of reflection : The angle which the reflected ray makes with the normal is known as the angle of reflection. It is denoted by r. ∠BON is the angle of reflection.
❐ Plane : A flat, two dimensional surface without any thickness is called a plane.
2. State the laws of reflection with a proper ray diagram.
= A ray of light which strikes a reflecting surface gets reflected back obeying certain laws. These laws are known as laws of reflection.
There are two laws of reflection which are as follows.
i. The angle of incidence is equal to the angle of reflection, i.e., ∠i=∠r.
ii. The incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane.
3. Draw and explain the image formation of a point object by a plane mirror.
= Consider a point object O placed in front of the reflecting surface of a plane mirror XY. The rays starting from O travel in all the directions. For our convenience, we will consider only two rays, say OA and OB. These two rays strike the mirror at positions A and B. They will then proceed in the directions AC and BD respectively, after reflection such that the angle of reflection is equal to the angle of incidence according to the laws of reflection.
The rays reflected from the mirror form the image. When we see an image in the mirror we are basically looking at the reflected rays. When these reflected rays reach our eyes, they appear to be coming form a point I behind the mirror. So the image of an object is formed behind the mirror, such that the distance of the object form the reflecting surface is same as the distance of the image from the reflecting surface (OX'=IX').
4. State the characteristics of the image formed by a plane mirror.
= The characteristics of the image formed by a plane mirror are -
❐ The image formed by a plane mirror is erect unlike the image formed by the pinhole camera which is inverted.
❐ The image formed by a plane mirror is of the same size as the object.
❐ The image is formed behind the mirror. Also, the distance of the image behind the mirror is same as that of the object in front of it. If the object is moved closer to the mirror, the image also more closer to the mirror.
❐ The image formed by a plane mirror is virtual because the rays of light do not actually meet at the point. We have extended the rays to meet at that point, so the rays appear to be coming from the point to out eyes. Also, the image formed by a plane mirror cannot be taken on the screen.
❐ The image formed by the plane mirror is laterally inverted.
5. Enlist a few uses of a plane mirror.
= A plane mirror has many uses in our life.
❐ Plane mirrors are used at home, in salons and beauty parlours.
❐ Soldiers use plane mirrors for signalling.
❐ Architects and interior designers use plane mirrors to give a feeling of space in smaller houses.
❐ Plane mirrors are also used in microscopes, telescopes, kaleidoscopes and other instruments.
❐ Plane mirrors are also used in solar cookers to reflect sunlight towards the box.
6. What is subtraction of colours? Explain.
= A transparent object (e.g., glass or cellophane sheet) allows light of certain colours to pass and blocks other colours by absorbing them. Hence, it acts like a colour filter. This is referred to as the subtraction of colours.
For example, a sheet of paper is made of a material that is capable of absorbing blue light. Therefore, when white light falls on this sheet, it appears yellow.
White - Blue = Red + Green + Blue - Blue
= Red + Green
= Yellow.
If magenta light falls on the same sheet of paper, it appears red.
Magenta - Blue = Red + Blue - Blue
= Red.
7. With the help of an example, explain the appearance of colour of opaque objects.
= Opaque objects are those which do not let light pass through them. These objects such as table, book and almirah are all around us. When white light falls on these objects they absorb light of few colours and reflect other lights. The colour of opaque objects that is visible to our eyes is the colour of light which these objects reflect.
Thus, appearance of colour of an object is due to the absorption and reflection of different colours by the object.
For example, a leaf appears green in colour in white light because a leaf absorbs all colours of white light but reflects green colour.
An object which absorbs lights of all colours, appears dark, i.e., black. This is because it does not reflect any colour.
In a similar way, an object which reflects lights of all colours and does not absorb any colour appears to be white in colour.
8. Write a short note on speed of light.
= Light travels in vacuum or air with a speed of 2.9979🇽10⁸ m s⁻¹ which is taken as 3🇽10⁸ m s⁻¹ for convenience. This means that light can travel a distance of about 299,792,458 metres in one second in vacuum.
The speed of light is the maximum in vacuum and, for any other transparent medium through which light passes, its speed slows down.
The value by which the speed of light decreases on entering a medium depends on the medium through which it passes. For example, the speed of light decreases more on entering glass as compared to when it enters water.
Review Your Learning
A. Answer in one word.
1. The angle between the incident ray and the normal.
= Angle of incidence.
2. The colour resulting form mixing two primary colours.
= Secondary colours.
3. The image that is obtained on a screen.
= Real image.
4. The reflection of light from a smooth surface.
= Regular reflection.
5. The return of light after striking a surface.
= Reflection of light.
B. State whether the given statements are true (T) or false (F).
1. The image formed by a plane mirror is virtual. [T]
2. A red rose appears white in white light. [F]
3. The green colour appears darker in red light. [T]
4. The image of left hand appears like that of a right hand in a plane mirror. [T]
5. The angle of incidence is equal to the angle of reflection. [T]
C. Match the columns.
=
|
Column A |
Column B |
|---|---|
|
1. Secondary colour. |
(a) Dispersion of light. [5] |
|
2. Real image. |
(b) Reflection of light. [4] |
|
3. Virtual image. |
(c) Magenta. [1] |
|
4. Plane mirror. |
(d) Erect. [3] |
|
5. Prism. |
(e) Inverted. [2] |
D. Explain the following terms.
1. Primary colours.
= The colours of light that cannot be obtained or created by mixing other colours.
2. Secondary colours.
= The colours of light that can be produced by mixing two primary colours.
3. Regular reflection.
= A reflection when a beam of light falls on a smooth highly polished surface such as the rays of the beam are reflected parallel to each other.
4. Dispersion of light.
= The phenomenon of splitting of the white light into several different component of light.
5. Rectilinear propagation of light.
= The property of light due to which it travels in a straight line.
E. Answer the following questions.
1. The angle of incidence on a plane mirror is 30°. Find the angle between incident ray and reflected ray.
= We know that angle of incidence and the angle of reflection are equal. So, here the angle between incident ray and reflected ray will be 60°.
2. What are the laws of reflection?
= There are two laws of reflection which are as follows.
i. The angle of incidence is equal to the angle of reflection, i.e., ∠i=∠r.
ii. The incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane.
3. What is the angle of incidence when a ray of light falls normally on a plane mirror?
= When a ray of light falls normally on a plane mirror, the angle of incidence is 0°.
4. How can you prepare a plane mirror?
= To make a plane mirror, a thin piece of glass is taken and coated with silver on its back surface to make it shiny.
5. What do you mean by a real image? How is it different from the virtual image?
= When the rays of light are reflected in such a way that they actually meet at some point on the screen, the image so formed is called real image.
Real Image |
Virtual Image |
|---|---|
1. It can be taken on a screen. |
1. It cannot be taken on a screen. |
2. It is always inverted. |
2. It is always erect. |
3. It is formed in front of the mirror. |
3. It is formed behind the mirror. |