Class-8 General Science Notes (Gaba Books)- Unit-13 Light and its Properties - SLO-Based Short and Brief Questions and Answers

Class-8 General Science Notes (Gaba Books)

Unit-13 Light and its Properties 

SLO-Based Short and Brief Questions and Answers

Q1. What is light?

Ans

Light is a form of energy, and it travels in straight lines. 

Q2. What is the speed of light in different mediums?

Ans.

Light travels at a very high speed from one place to another. The speed of light decreases in a denser medium while it increases in a rarer medium. Scientists have found that:

• Light travels with the highest speed in air or vacuum. The speed of light in air or vacuum is about 300,000 km/s. 
• In water, light travels at a speed of about 225,000 km/s.
• The speed of light in glass is reduced to only about 200,000 km/s. 

Q3. What is critical density?

Ans.

Critical density is a property of a transparent material; it is a measure of the speed of light through the material. The speed of light decreases in a denser medium while it increases in a rarer medium.

Q4. What is the refraction of light?

Ans.

The phenomenon of refraction takes place when light rays pass from one medium to another medium, they change their direction and this change in the direction of light rays is called refraction of light.

Q5. What will be the change in the direction of light when passing through the Normal?

Ans.

There is no change of direction when the light rays pass through normal (i.e. in a direction perpendicular to the surface) into the other medium. 

Q6. What is an index of refraction or refractive index

Ans. 

The refractive index is the measure of the bending of a light ray when passing from one medium to another. Mathematically refractive index is the ratio of the speed of light in vacuum to that in the medium and is denoted by μ. It has has no unit.

Table showing refractive index through different mediums

★ Substance:	★ Refractive Index:
1) Water	= 1.33
2) Glass	= 1.5 TO 1.9
3) Glycerine	= 1.47
4) Ice	= 1.31
5) Kerosene Oil	= 1.39
6) Turpentine Oil	= 1.47

Q7. State laws of refraction? 

There are two Laws of Refraction:

1. The incident ray refracted ray, and the normal to the interface of two media at the point of incidence all lie on the same plane.

2. The ratio of sine of angle of incidence to the sine of angle of refraction is equal to the refractive index of second medium with respect to first medium.

Q8. Define the following.

Incident ray: The ray of light falling on the surface is called the incident ray. 

Angle incidence: The angle between the incident ray and the normal is called the angle incidence 

Refracted ray: The ray which has changed its direction is called the refracted ray. 

Angle of refraction: The angle between the refracted ray and the normal is called the angle of refraction.

Normal: It is a line drawn perpendicular to the surface of the second medium at the point where the light rays strike the surface of the second medium. It makes a 90° angle with the surface.

Q10. What happens when light travels from rarer medium to denser medium?

Ans. 

If a ray of light travels from a rarer medium into a denser medium, it is refracted toward the normal at the point of incidence. The angle! refraction (r) is less than the angle of incidence.

Q11. What happens when light travels from denser medium to rarer medium?

Ans. 

If a ray of light travels from a denser medium into a rarer medium, it is refracted away from the normal at the point of incidence. The angle! refraction (r) is more than the angle of incidence.

Q12. What happens when light incident perpendicular on surface of a medium?

Ans. 

When a ray of light falls normally on the surface separating two substances passes straight through without refraction. 

Q13. Who discovered and explain the phenomenon of refraction of light?

Ans. 

Willebrod Snell (1580-1626) was a Dutch mathematician who discovered and explained the phenomenon of refraction of light. 

Q14. What is prism? Explain refraction through a prism.

Ans.

A prism is a wedge-shaped block of glass. Two plane surfaces through which a light ray is passed are called the refracting faces of the prism and the angle between them is known as the refracting angle of the prism. The face opposite to the angle of the prism is called the base of the prism. When a ray of light passes through a prism, it splits into its component lights and this phenomenon of splitting of white light into different coloured lights is called dispersion of light.

Q15. What is the dispersion of light?

Ans.

The phenomenon of splitting white light into different coloured lights is called dispersion of light. The rainbow is a very common example of dispersion of light in nature.

Q16. Explain the dispersion of light through a prism

Ans.

Allow a very narrow beam of white light to fall on one face of a prism. Hold a white screen on the other side so that light emerging out of the other face falls on it. It is observed that the light has split up into different colours on the screen. The seven colours were observed in the following order: violet, indigo, blue, green, yellow, orange and red. These colours can be remembered by the word VIBGYOR. Violet is most deviated and red is least deviated. 

Q17. What is a spectrum of light?

The band of seven colours obtained on a screen when white light passes through a prism and splits into its constituent colour is called spectrum of light. Thus, white light is a mixture of seven lights of different colours,

Q18. How rainbow is formed? 

Ans.

After rain, tiny droplets of water remain suspended in the atmosphere. When the sun appears, the light gets dispersed by refraction through these drops of water, As a result, a rainbow appears in the sky, comprising of seven colours.

Q19. Why do we see different coloured objects?

Ans.

Coloured objects absorb all the light rays of the spectrum except the light ray that shows their colours. Thus, they reflect only this light ray which appears as the colour of their objects. For example, a blue-coloured object looks blue because it reflects blue and absorbs all other colours.

Q20. What is a lens? classify lens.

Ans.

A transparent material bounded by two surfaces of which at least one is spherical is called a lens. Lenses are of various shapes and sizes. They can, however, be classified into two types: converging (or convex) lens, and diverging (or concave) lens.

Q21. What is a convex lens?

Ans. 

A converging (convex) lens is thick in the middle and becomes thinner at the edges.  A magnifying lens is a converging lens. A converging lens bends a ray of light passing through it inwards.

Q22. What is a concave lens?

Ans. 

A diverging (concave) lens is thin in the middle and thicker at the edges. It has the property of bending a ray of light passing through it outwards.

Q23. Define the following terms related to lens?

Principal axis: The line joining the centres of the two spheres (C1 and C2) from which the lens is made is called the principal axis of the lens.

Optical centre: The optical centre of a lens is a point on the principal axis from which the passing ray does not undergo any deviation. 

Principal focus: The point on the principal axis of a lens to which parallel rays of light converge or from which they appear to diverge after refraction.

Focal length: The distance between the optical centre and the focus of a lens is called the focal length of the lens. The thicker the lens, the more it bends light rays, and the smaller is its focal length.

Power of a lens

The Power of a lens is its ability to converge or diverge the rays of light falling on it and is defined as the reciprocal of the focal length of a lens (in meters). The Unit of power of a lens is dioptre (D).

Q24. Define a diopter.

Ans. 

A diopter is the power of a lens whose focal length is 1 meter. Its vale is positive for a convex lens and negative for a concave lens.

Q25. What is a real image?

Ans. When two or more rays from the object, after refraction through the lens, actually meet at the point. A real image is said to be formed. Or. All images that can be obtained on a screen. It is always inverted.

Q26. What is a virtual image?

Ans. 

When two or more rays starting from an object, after refraction through the lens, do not actually meet but appear to meet from a point one side of the object, the virtual image is said to formed. It cannot be obtained on a screen. It always erect and formed on the same side of e lens on which the object is situated.

Q27. What will happen when a ray incident on the lens parallel to the principal axis: 

Ans.

When a light ray incident on the lens parallel to the principal axis, this will pass through the focus in a converging (convex) lens.  In the case of a diverging (concave) lens, it will appear to come from the focus.

Diagram:

Q28. What will happen when a light ray passing through the optical centre?

Ans.

A ray passing through the optical centre: This will emerge un-deviated in a converging lens. And a ray passing through the focus: This will emerge parallel to the principal axis in a converging lens.

Q29. Explain the images formed by a convex lens. (Long)

Ans. 

IMAGES FORMED BY A CONVEX LENS: 

The image formed by a convex lens varies in size, position and nature (erect or inverted, real or virtual) depending upon the distance of the object from the lens.

Exercise Q/A

Answer the following questions:

Q1. Describe an image formed by the convex lens when an object is placed very far off?

Ans.

Object very far off : The rays of light coming from an object that is very far off (like the sun) are parallel to each other. The image is formed at the focus.  It is real, inverted and smaller than the object.

Q2. Describe an image formed by the convex lens when an object is placed beyond 2F?

Ans.

Object beyond 2F: The image formed is between F and 2F on the other side of the lens. It is real, inverted and smaller than the object.

Q3. Describe an image formed by the convex lens when an object is placed beyond 2F?

Ans.

Object at 2F: The image is real, inverted and of the same size as the object. It is located at 2F on the other side of the lens.

Q4. Describe an image formed by the convex lens when an object is placed between F and 2F?

Ans.

Object between F and 2F: The image is real, inverted and larger than the object. It is situated beyond 2F on the other side of the lens.

Q5. Describe an image formed by the convex lens when an object is placed between at F?

Ans.

Object at F: When the object at Focus (F), the refracted rays emerge parallel to each other. Therefore, no image is formed as the rays do not meet However, since parallel rays are assumed to meet at infinity, we say that the image is formed at infinity. It is real, inverted image, very much bigger in size than the object.

Q6. Describe an image formed by the convex lens when an object is placed between at F and O?

Ans.

Object between F and O: The rays diverge after refraction. Thus, a real image is not formed in this case. The image is, therefore, virtual. It is erect, bigger than the object and is formed on the same side of the lens as the object 

Q7. Describe images formed by a concave lens.

Ans.

The rays falling on a concave lens, after refraction, always diverge. Therefore, no matter where the object is kept, a diverging lens always forms a virtual image, which is erect and smaller in size than the object. It is formed between the optical centre and the focus, on the same side of the lens as the object

Q8. Write the name of some optical instruments in which lenses are used?

Ans.

Lenses are used in several optical instruments such as cameras, microscopes, telescopes slide projectors and binoculars. 

Q9. where is image formed in our eye?

Ans.

The eye is a natural optical instrument. It enables us to see things around us by forming their images on a screen called the RETINA.

Amazing facts!

Our eyes have a biconvex lens inside that forms the real and inverted image on retina. This image is sent to the brain which shows this image as upright. Cones and rods are sensory cells of retina that help to see colored light and dim light respectively. The retina has 107 million light sensitive cells.

KEY POINTS

Here is a list of key points to remember:

The velocity of light is different in different media.

In air or vacuum light travels at a speed of about 300,000 km/s:

The speed of light in other transparent substances is less than that in air. These

Look at the pencil in this photograph. We know the pencil is straight, but it looks bent. Our eyes are being tricked into seeing something that is not true. This happens because light travels more slowly through the water than it does through the air. As a result, when the rays cross the boundary between the water and the air, they change direction. This change in direction is called refraction. We say that the rays have been refracted.

Let us understand the effect of the change of medium on the direction of propagation of light by performing the following activities.

Q. How you will show the refraction of light in water.

Ans. 

Taking an empty cup or beaker and place a coin at its bottom. Now, position yourse such that the coin is just not visible and is obstructed by the edge of the cup (Fig. 13.1). Now, ask someone to gradually pour water into the cup without disturbing the coin. What do you observe? You will observe that as the water level rises, the coin appears in sight.

You know that you can see an object because rays of light from the object reach your eyes.

When the coin is not visible to you, the rays of light travelling in a straight line do not enter your eyes (Fig. 13.1). As soon as water is poured in the cup, the rays change the direction and you can see the coin.

Fig. 13.2 shows you what happens to two rays of light when they leave the water and enter air-they change directions. They now appear to be coming from position K instead of R. The coin, thus, appears to be raised to K and becomes visible to you.

Good to know

substances are said to be optically denser than air

The bending of light ray when it passes from one transparent medium to another is called refraction of light.

AA ray of light bends towards the normal when it passes from a rarer medium to a denser medium, and bends away from the normal when it travels from a denser medium to a rarer medium.

A Refractive index is the ratio of the speed of light in vacuum to that in the medium and is denoted by p

There are two laws of refraction.

A White light consists of seven colours which can be remembered by the word VIBGYOR The band of seven colours is called spectrum. The red rays are least deviated and the violet is most deviated.

The phenomenon of splitting up of white light into different colours is called dispersion of light A lens is a transparent medium bound by two surfaces of which at least one is spherical Lenses are of two types: convex or converging lenses and concave or diverging lenses. A convex lens forms virtual or real image depending on the position of the object. A concave lens forms only virtual images for all positions of the object. The power of a lens is the reciprocal of its focal length expressed in metres. The S.1 unit of power of a lens is diopters.

The position, size and nature of the image formed by a lens depends on the distance of the object from the lens and the focal length of the lens.

Lenses are used in several optical instruments such as cameras, telescopes, microscopes, spectacles and binoculars.