WBBSE Solutions For Class 8 School Science Chapter 1 Physical Environment Light Short Answer Questions

Chapter 1 Physical Environment Short Answer Type Questions

Question 1. What do you mean by a real image and virtual image?
Answer:

Real Image And Virtual Image:-

Real images are formed when reflected rays or refracted rays actually meet at some point. Virtual images are formed when the reflected or the refracted rays appear to diverge from some point.

Question 2. What do you mean by the refraction of light?
Answer:

Refraction Of Light:-

When light is traveling from one medium to the other, it deviates from its original path at the surface of separation between the two mediums. The change of direction of the incident light from its original direction is called the refraction of light.

Question 3. What do you mean by absolute refractive index?
Answer:

Absolute Refractive Index:-

When refraction occurs between a vacuum and a certain medium, then the refractive index of the medium is called Absolute Refractive Index. This is the ratio of the velocity of light in a vacuum to the velocity of light in that medium.

Question 4. Can the absolute refractive index of a medium be less than 1 ?
Answer: Absolute refractive index of a medium = velocity of light in vacuum/ velocity of light in the medium. Since the velocity of light is maximum in a vacuum, so absolute refractive index of a medium can never be less than 1.

Question 5. Write the laws of refraction.
Answer:

Laws Of Refraction:-

There are two laws of refraction. They are as follows:

First Law: The incident ray, the refracted ray, and the normal drawn at the point of incidence on the refracting surface lie on the same plane.

Second Law: For refraction of an obliquely incident ray of light of any given color in a given pair of optical media, the refractive index is constant, irrespective of the magnitudes of the angle of incidence and angle of refraction.

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Question 6. On which factors does the refractive index depend?
Answer:

Refractive index depends on the nature of two media and on the color of light. For example, when a ray of light travels from an optically denser medium to an optically rarer medium, then the magnitude of the refractive index for the light of different colors will be in the order: of red< green < blue < violet.

Question 7. Inversion of the image of an object formed by a plane mirror occurs left for right and vice versa. Why does it not occur top for the bottom?
Answer:

The straight line joining any point on an object and its corresponding mirror image always meets the mirror perpendicularly.

If the inversion of an image happens top for the bottom, the straight line, connecting a point of the object with its image would meet the mirror obliquely.

Thus, a plane mirror does not produce inversion of any extended non- sym metric object top for bottom or vice versa.

Question 8. Describe appropriately, the refraction of light when a ray of light travels from an optically rarer medium to an optically denser medium.
Answer:

When a ray of light travels from an optically rarer medium, such as air, to an optically denser medium (say glass),

the ray in the denser medium moves towards the normal, and the angle of incidence is always greater than the angle of refraction (as shown in ). Here ∠AON is the angle of incidence;

∠BON’ is the angle of refraction; AO is the incident ray; OB is the refracted ray and NN’ is normal.

WBBSE Solutions For Class 8 Chapter-1 Physical environment Sec-4 Light Refraction

Question 9. Describe appropriately, the refraction of light when a ray of light travels from an optically denser medium to an optically rarer medium.
Answer:

When a ray of light travels from an opt it ally denser medium, such as glass, to an optically rarer medium (say air),

then the ray of light in the rarer medium moves away from the normal and the angle of incidence is always less than the angle of refraction (as shown in).

Here Z AON is the angle of incidence; Z BON is the angle of refraction; AO is the incident ray; OB is the refracted ray and NN’ is normal.

Question 10. What do you mean by critical angle for a given pair of media?
Answer:

Critical Angle For A Given Pair Of Media:-

When a ray of light of any given color tends to travel from an optically denser medium to an optically rarer medium, then for a certain angle of incidence the angle of refraction is 90°.

The corresponding angle of incidence is the critical angle of the given pair of media for the given color of light.

WBBSE Solutions For Class 8 Chapter-1 Physical environment Sec-4 Light Light tarvels

Question 11. What do you mean by total internal reflection?
Answer:

Total Internal Reflection:-

When a ray of light tends to travel from an optically denser medium to an optically rarer medium,

then if the angle of incidence (∠DON exceeds the critical angle (6C), the incident light totally reflects back along OR that is to the first medium (optically denser medium)

Such a phenomenon is called total internal reflection. The term “total” is used because the incident light totally reflects back from the surface of separation of the two media and no part of the incident light is either absorbed by the medium or refracted.

WBBSE Solutions For Class 8 Chapter-1 Physical environment Sec-4 Light Internal reflection

Question 12. What are the primary conditions to be fulfilled so that total internal reflection can occur?
Answer:

The primary conditions for total internal reflection to occur are – the ray of light must pass from an optically denser medium to an optically rarer medium and

the angle of incidence in the optically denser medium must be greater than the characteristic critical angle of the two media.

Question 13. Explain why a diamond or a gemstone glitters and looks very bright.
Answer:

Usually, diamonds and other gems are constituted of materials with a high refractive index. The critical angle of each of which with respect to air is thus very small.

For example, the critical angle of a diamond is only 24.5°. Also, diamond or any other gem is cut in such a way that, light can get into it through all surfaces but can emerge from very few surfaces.

This is because, the diamond or the gemstone is cut in such a way, that the rays within the “body” trying to come out are incident on most of the surfaces at an angle exceeding the critical angle.

After undergoing several total internal reflections, the light rays are incident on a small number of surfaces at angles less than the critical angle and emerge from those surfaces only.

Hence, the emergent light is very intense and that is why a diamond or a gem looks very bright.

Question 14. Explain why a crack in the glass of a window pane looks shiny.
Answer:

A crack in the glass of a window pane looks shiny. Some air is present in the gap of a crack in the glass. So, light rays passing through the denser medium (i.e. glass)

when tends to pass through the rarer medium (i.e. air), total internal reflection occurs at some point in the crack. Hence those points of the crack look shiny.

Question 15. Explain why an empty test tube dipped in the water looks shiny.
Answer:

An empty test tube dipped in the water looks shiny. An empty test tube is dipped in water an inclined way.

Light rays passing through water outside the tube tend to pass through air present in the empty test tube.

Thus light passes from an optically denser medium to an optically rarer medium. At some points on the surface of the test tube, rays of light are incident at angles exceeding the critical angle of water to air.

At those points total internal reflection takes place and so the empty portion of the test tube looks bright when viewed from above vertically.

WBBSE Solutions For Class 8 Chapter-1 Physical environment Sec-4 Light Tude dipped

Question 16. Explain why a drop of water on an arum leaf seems glittering.
Answer:

A drop of water on the arum leaf seems glittering. This is because when a ray of light travels from inside the water droplet to the air,

the angle of incidence exceeds the critical angle of the two media (i.e. water and air).

So, total internal reflection occurs at the surface of the separation between water and air. When the emerging ray of light reaches to viewer’s eye, the viewer finds the area glittering.

Question 17. A fish of length 8 cm is lying parallel to the surface of the water at a depth of 4cm below the surface. If it is viewed perpendicularly from the top, then how long would it appear? Explain with reasons.
Answer:

A ray incident normally on the surface of separation of two media does not undergo refraction and passes undeviated in the second medium from the first medium.

Hence when viewed down normally from above the surface of the water, the fish would appear 8 cm in length.

Question 18. 1. If the angle of incidence of a ray of light on a plane mirror is 60° then what is the angle of deviation?
2. If the distance between the object and its image in a plane mirror is 120 cm then what is the distance between the object and the mirror?
Answer:

1. <i=<r=60°
Angle of deviation
=180°-(<i+<r)
=60°

WBBSE Solutions For Class 8 Chapter-1 Physical environment Sec-4 Light Angle of deviation

2. Distance between object 5 and the plane mirror =120/2=60 cm.

Question 19. A boy walks at a speed of 2 km/hr toward a mirror. What will be their speed of his? Let the boy be x km away from the mirror at the beginning.
Answer:

Given That

A boy walks at a speed of 2 km/hr toward a mirror.

Therefore, his image was also x km away from the mirror and the distance between the boy and his image was 2x km. The distance of the boy from the mirror after 1 hour = (x-2) km.

∴Distance of his image from the mirror after 1 hour = (x-2) km.

Distance covered by image w.r. to mirror in 1 hour = x – (x-2) = 2 km.

∴Speed of image w.r. to the mirror=2 km/hour. Distance between the boy and his image after 1 hour = 2 (x – 2) km = 2x – 4 km.

Change of Distance of the image w.r. to the boy in 1 hour = 2x – (2x-A) = 4 km.

∴Speed of image w.r. to the boy = 4 km/hour.

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