Living Science Physics Class 7 ICSE Solutions Chapter 4 Light

ICSE Living Science Physics for Class 7 Solutions Chapter 4 Light

Think And Answer (Page 47)

Why is the word ‘AMBULANCE’ printed laterally inverted on an ambulance van?
Answer:
The word AMBULANCE is printed laterally inverted on an ambulance van so that it reads correctly in the side view mirrors of the vehicles in front of the van. This would allow the vehicles to recognize an ambulance van easily and they could then ensure that the ambulance has enough space to move ahead.

Check Your Progress (Page 48)

A. Write true or false. Correct the false statements.

Question 1.
Regular reflection gives diffused or blurred images.
Answer:
False. Regular reflection gives clear images.

Question 2.
A ray of light obtained as a result of reflection from a surface is called an incident ray.
Answer:
False. A ray of light obtained as a result of reflection from a surface is called .a reflected ray.

Question 3.
In reflection, the angle of incidence is always greater than the angle of reflection.
Answer:
False. In reflection, the angle of incidence is always equal to the angle of reflection.

Question 4.
The Image formed by a plane mirror is of the same size as the object.
Answer:
Truc.

Question 5.
The behaviour of mirrors due to which sides of an object gets inversed is called lateral inversion.
Answer:
True.

B. Find the unknown angle.

Answer:
1. 60°
2. 35°

A. Tick the most appropriate answer.

Question 1.
The phenomenon by which the incident light falling on a surface is sent back into the same medium is known
as
a. lateral inversion
b. reflection
c. polarization
d. dispersion.
Answer:
b. reflection

Question 2.
Which type of reflection will take place when light is incident on a highly polished surface ?
a. Regular
b. Diffused
c. Irregular
d. normal.
Answer:
a. Regular

Question 3.
A mirror that has a flat reflecting surface is called a
a. flat mirror
b. spherical mirror
c. plane mirror
d. Normal
Answer:
c. plane mirror

Question 4.
An imaginary line drawn perpendicular to the mirror at the point of incidence is called the
a. normal
b. reflected ray
c. incident ray
d. refracted ray
Answer:
a. normal

Question 5.
If the angle of incidence for a ray of light is 40°, then its angle of reflection will be
a. 20°
b. 80°
c. 40°
d. 0°
Answer:
c. 40°

Question 6.
If you stand 1 m in front of a plane mirror, how far away would you see yourself in the mirror ?
a. 1 m
b. 2 m
c. 3 m
d. 4 m
Answer:
a. 1 m

Question 7.
The word ‘B’ appears in a plane mirror as ‘8’ due to
a. reflection
b. wrong positioning of the mirror
c. lateral inversion
d. colour addition
Answer:
c. lateral inversion

Question 8.
Which of the following is not a primary colour?
a. red
b. green
c. black
d. blue
Answer:
c. black

Question 9.
Which of the following combination of colour will produce white colour?
a. green + magenta
b. cyan + blue
c. yellow + red
d. red + green
Answer:
d. red + green

Question 10.
A red rose when seen in blue light appears
a. red.
b. blue.
c. black.
d. green
Answer:
c. black.

B. Fill in the blanks.

Question 1.
Regular reflection results in the formation of …………. images.
Answer:
Regular reflection results in the formation of clear images.

Question 2.
A mirror is made by …………… one side of a thin sheet of glass.
Answer:
A mirror is made by silvering one side of a thin sheet of glass.

Question 3.
A ray of light that falls on a mirror is called the ………….. ray.
Answer:
A ray of light that falls on a mirror is called the incident ray.

Question 4.
The law of reflection states that the angle of incidence is always ………….. to the angle of reflection.
Answer:
The law of reflection states that the angle of incidence is always equal to the angle of reflection.

Question 5.
The speed of light is around in vacuum.
Answer:
The speed of light is around 3 x 10® m/s in vacuum.

Question 6.
When the lights of red, green and blue colours are mixed, light is produced.
Answer:
When the lights of red, green and blue colours are mixed, white light is produced.

Question 7.
A pair of colours which on mixing produce white colour are called …………. colours.
Answer:
A pair of colours which on mixing produce white colour are called secondary colours.

C. Write true or false. Correct the false statements.

Question 1.
The regular nature of a reflection can be determined from the clarity of the image formed.
Answer:
True.

Question 2.
Light needs a material medium to travel.
Answer:
False. Light does not needs a material medium to travel.

Question 3.
The white object seen in a coloured light appears to have the colour of the coloured light.
Answer:
True.

Question 4.
A magenta colour object when seen in yellow light appears red.
Answer:
False. A magenta colour object when seen in yellow light appears black.

Question 5.
The colour of an object does not depend upon the colour of light reflected by it.
Answer:
False. The colour of an object depends upon the colour of light reflected by it.

Question 6.
The colours red, green and blue are called secondary colours.
Answer:
False. The colours red, green and blue are called primary colours.

D. Answer the following in short.

Question 1.
State the differences between regular and diffused reflection.
Answer:
Differences between regular and diffused reflection
Regular reflection :

• When a parallel beam of light strikes a smooth and , polished surface and all the , light rays are reflected as
  a parallel beam of light in a definite direction, it is called a regular reflection.
• It results in the formation of clear images.
• Examples : Mirrors, still water surfaces, oiled surfaces and polished surfaces cause regular reflection.

Diffused reflection:

• When a parallel beam of light strikes a rough surface, the light rays are reflected in different directions and get scattered, it is called irregular or diffused reflection.
• It results in the formation of diffused or blurred images.
• Examples : Wavy water surface or old chipped off mirrors cause diffused reflection.

(iv) Regular reflection :

Diffused reflection:

Question 2.
If the angle between the incident ray and a plane mirror is 30°, then what will be the angle of reflection?
Answer:
The angle of reflection will be 30°.

Question 3.
State any two uses of a plane mirror.
Answer:
Uses of plane mirrors are :

• Plane mirrors are used as looking glasses.
• They are used in periscopes and kaleidoscopes.
• Army persons use plane mirrors for signalling.
• They are also used in optical instruments and box-type solar cookers to reflect sunrays towards the vessel to be heated.

Question 4.
What is meant by lateral inversion of an image formed by a plane mirror ?
Answer:
When we observe ourselves in a plane mirror, we see the left of our body becomes the right of our body. This behaviour of plane mirrors according to which the sides of an object gets , reversed is called lateral inversion. In other words, the right of the object becomes the left of the image and vice, versa. Due to lateral inversion of the image formed in a plane mirror ‘ it becomes difficult to read the text reflected by it. The letter B looks like a, D will look like Q, L will look like J and so on.

Question 5.
Write a short note on the speed of light.
Answer:
Light rays always travel in a straight line. Unlike sound, light j does not need a material medium to travel. Light can travel 5 through vacuum. The speed of light is around 3,00,000 km/s i in vacuum.

Question 6.
Name the primary colours. Why are these colours called so?
Answer:
The colours red, green and blue (RGB) are called primary colours. These colours are called primary colours, because they cannot be made by mixing other colours.

Question 7.
The beams of red light and green light fall on the same spot on a white screen. What colour will appear on the screen?
Answer:
Yellow colour will appear on the screen.

E. Answer the following in detail.

Question 1.
Describe an experiment to show that the angle of incidence is equal to the angle of- reflection.
Answer:
Aim : To verify the laws of reflection
Materials Required : A plane mirror, a pencil, some pins, A4 – sized white paper, drawing board and a protractor.
Procedure : Fix the paper on the drawing board. Near the longer side of the drawing paper, draw a straight line AB and mark its mid-point as O. Draw a perpendicular ON to AB at O. Using a protractor, draw angle XON equal to 60°.

Place the long face of the plane mirror strip and use plasticine to hold the mirror in a vertical position. Fix two pins P and Q on line XO. The pins should be at least 5 cm apart and in an upright position. Looking into the mirror from the side of the perpendicular ON, fix two more pins, R and S, in such a way that pin R, pin S and the respective images of pins P and Q appear to be in the same straight line.

Remove the mirror and the pins one by one. Draw small circles around each pinhole, using a pencil to mark their respective positions. Join RS and produce the line to meet AB at O. Let this line ‘ be YO. From point P, draw a perpendicular to AB to meet AB at L and YO at P₁ (when extended). Measure the angle YON.

Observation : Z XON is equal to z YON, that is, angle of incidence is equal to angle of reflection. You will also observe that the incident ray XO, the normal NO and the reflected ray YO all lie in the plane of the paper used, that is, in the same plane.

Question 2.
Prove geometrically that the image formed by a plane mirror is at the same distance behind the mirror as the objects in front of the mirror.
Answer:
To understand the reflection from a plane mirror and the image formed due to it, let us consider a point object O kept in front, of the plane mirror strip, MM’.

The rays of light travel out in all directions from O. Consider two light rays OP and OQ from O (point object) and incident on the mirror at points P and Q, respectively. Let these two rays be reflected along PA and QB. Draw the normals PN and QN to the mirror at points P and Q, respectively. As per the laws of reflections, angle of incidence is equal to the angle of reflection for each incident ray and its corresponding reflected ray. Thus,
∠i₁ = ∠r₁
And ∠i₂ = ∠r₂

If the reflected rays PA and QB are produced backwards, they meet at a point I, behind the mirror, from where rays seem to be coming and image of the object O is formed. As the reflected rays PA and QB do not actually meet at I but only appear to do so, the image is said to be virtual. Draw a perpendicular from point I to the mirror MM’ to meet MM at S and extend it to meet point O. By measuring the length of OS and SI with a ruler, you will see that OS = SI
i. e., for a plane mirror, the image formed is at the same distance behind the mirror as the object in front of the mirror.

Question 3.
State the characteristics of an image formed by a plane mirror.
Answer:
Characteristics of an image formed by a plane mirror.

1. An image formed by a plane mirror is always virtual, i. e., it cannot be obtained on a screen.
2. The image formed is at the same distance behind the mirror as the object is in front of the mirror.
3. The image formed is of the same size as the object,
4. The image formed is laterally inverted.
5. The image formed is always erect.

Question 4.
What are secondary colours? Why are they called so?
Answer:
When any two primary colours are mixed in equal proportions, new colours are produced. The colours produced by mixing any two primary colours are called secondary or composite colours. Yellow, cyan and magenta are the three secondary colours. They are called secondary colours as they are produced by mixing primary colours.

Question 5.
How are we able to see different colours?
Answer:
We see objects due to the reflection of light falling on them. The colour of an object depends upon the colour of the light reflected by it. The colour of the reflected light depends upon the colour of the light falling on the object.

Question 6.
An object appears blue in white light. Of what colour will it appear in red light?
Answer:
The object appears black because no light is reflected from it.

Question 7.
What do you understand by colour subtraction?
Answer:
Colour subtraction means to reflect or absorb a particular colour of light in the presence of other colours of light. For example, a shirt is made of a material that is capable of absorbing green light. So, when white light falls on this shirt, if appears magenta.

F. Give reasons for the following.

Question 1.
Diffused reflection takes place on rough surfaces.
Answer:
Rough surfaces give diffused reflection because the parallel beam of light that strikes the rough surface gets scattered and are reflected in different directions.

Question 2.
Images formed by plane mirrors are always erect.
Answer:
The image formed by plane mirrors is virtual and virtual image is always erect.

Question 3.
Plane mirrors can be used for decorative purposes.
Answer:
Plane mirrors are used in kaleidoscope which is used to produce changing patterns or sequences by means of multiple reflections by many mirrors.

Question 4.
If we raise our left hand in front of a plane mirror, the image will raise its right hand.
Answer:
It is due to the lateral inversion, a behaviour shown by mirrors according to which the object and the image interchange their left and right.

Question 5.
A red rose when seen in green light appears black.
Answer:
When a coloured light falls on a different coloured object, the object appears black because no light is reflected from it. So, a red rose when seen in green light appears black.

G. In the given figure, PQ is a mirror strip, AO is a ray incident at O. Label the different terms related to reflection in the figure.
Answer:

Answer:

H. Locate the position of the image in the following figures.
Answer:

I. Look at the following figures and identify the colour of appearance of each surface.
Answer:

1. Yellow
2. Cyan
3. Blue
4. Red

Think And Answer

Question 1.
What happens when light falls on –
a. an opaque object
b. a highly polished surface?
Answer:
a. When light falls on an opaque object the path of the light will be blocked by the object.
b. When light falls on a highly polished surface, the light will be reflected completely by the surface.

Question 2.
The image formed by a plane mirror can be projected onto a screen. Do you agree? Give reason.
Answer:
No, the image formed by a plane mirror cannot be projected onto a screen as such images are virtual in nature.

Question 3.
Under very dim light, we are able to see the objects but cannot distinguish between colours. Why?
Answer:
Under very dim light, lights of different colours are mixed in very small proportions.

Living Science Physics Class 7 ICSE Solutions