1. What is meant by refraction of light?
Ans: The bending of light when it passes from one medium to the other is called refraction of light.
2. What is meant by real depth of a pond?
Ans: The actual depth of a pond is called its real depth.
3. What is meant by apparent depth of a pond?
Ans: Apparent depth is the virtual depth of a pond where the image of the bottom of the pond is seen due to the refraction of light.
4. How is refractive index related with real and apparent depth?
Ans: Refractive index is the ratio of real depth to the apparent depth.
5. What is meant by refractive index of glass is 1.5?
Ans: It means that the speed of light in glass is 1.5 times slower than the speed of light in vacuum.
6. What is total internal reflection?
Ans: The phenomenon of reflection of light to the original denser medium when the light passes from a denser medium to rarer medium at an angle of incidence greater than the critical angle is called total internal reflection.
7. What are the two conditions for total internal reflection?
Ans: The two conditions for total internal reflection are the ray of light must pass from a denser medium to the rarer medium and angle of incidence must be greater than the critical angle.
8. What are the advantages of totally reflecting prism over a plane mirror ?
Ans: A totally reflecting prism reflects all of the light and produces a single bright image but a plane mirror absorbs some light as well as multiple images with less brightness are formed.
9. Why is light not dispersed on passing through a glass slab?
Ans: It is because when a white ray of light enters the glass slab, the ray splits into its constituents colours inside the glass slab and when the different coloured rays come out to the air from the other end the rays combines to form a white ray again.
10. Why does a straight pencil partially dipped in water seem bending?
Ans: A straight pencil partially dipped in water seems to bend because of refraction of light.
11. Define denser medium.
Ans: The medium in which the velocity of light is less is known as denser medium.
12. Define rarer medium.
Ans: The medium in which the velocity of light is more is called rarer medium.
13. Define the refractive index of a medium.
Ans: The ratio of the velocity of the light in vacuum or air to the velocity of light in the given medium is called the refractive index of that medium.
14. What is an optical fiber?
Ans: An optical fiber is a very thin, flexible glass or quartz rod which can carry information in the form of wave by the process of total internal reflection.
15. What is the cause of dispersion of light?
Ans: Dispersion of light is due to the variation in the wavelength or speed when it enters from one optical medium to another.
16. Write the reason
I. A diamond sparkles more than glass cut to similar shapes.
II. From bottom of aquarium, we see fish above the water surface.
Ans: I. A diamond sparkles more than glass cut to similar shapes due to the total internal refraction of light rays. The refractive index of diamond is the largest with smallest critical angle of about 24°C. Light rays which enter the diamond experience total internal reflection repeatedly at different faces, and come out through one or two of faces.
II. From bottom of aquarium, we see fish above the water surface. This occurs due to the phenomenon of refraction of light.
17. A coin placed at bottom of a beaker containing water seems to be raised, why?
Ans: When a coin is placed at bottom of a beaker containing water, rays of light originate from the coin and passes through water air interface. At the water air interface, the light bends because light ray is passing from one medium to other medium i.e. denser medium (water) to rarer medium. Then, this bent rays of light incident on the observer's eyes. Now the observer who is seeing this from air, he is not going to be able to observe the bent path of light, the zigzag path the light rays are taking. The observer always has tendency to see things in the straight line. That is why the observer will see the light coming from a straight path. Observer observes the virtual image of coin formed above the exact position of coin, instead of real position of coin.
Let us consider a coin 'O' is placed at the bottom of the beaker containing water as shown in figure. Two rays OA and OD from O (coin) are refracted along AB and DE respectively. These divergent rays appear to come from 'I' which is the virtual image of 'O'. Clearly the image 'I' is at a less depth than 'O'. Thus the coin at 'O' appears to rise at I when the coin is kept in water as a result of refraction of light.
18. What is the critical angle?
Ans: The angle of incidence in the denser medium for which the angle of refraction in the rarer medium is 90o is called the critical angle.
19. What is the use of light pipe ?
Ans: Light pipe is used by doctors to study the interior parts of a human body which cannot be seen directly.
20. What is a light pipe?
Ans: A light pipe is a bent tube through which light travels in a curved path.
21. What is endoscopy?
Ans: Endoscopy is the practice of viewing internal parts of a patient such as a stomach using a light pipe.
22. Study the given diagram of dispersion light and answer the following questions:
Name the colour of rays A and B.
Why does the ray B bend more than A?
What happens when another prism is kept inverted between the prism and screen? Write with reason.
Ans: The colour of ray A - Red
The colour of ray B - Violet
White light is composed of seven different colour. In a vacuum (or air) all constituent colours (VIBGYOR) of white light have the same speed (i.e. 3 х 108). When white light enters a denser medium (prism), the speed of light decreases. The speed of the seven different colours of the white ray decreases at a different rate. It is a known fact that red light has the maximum speed and violet light has the minimum speed within a denser medium (prism). Thus, the red light (A) deviates the least and violet light (B) deviates the most.
When another prism is kept inverted between the prism and screen, the first prism disperses the white light into seven different colours, and the second prism will again combine, those seven dispersed colours to form the white light as shown in the diagram below:
Fig: Recombination of spectrum colours by a prism
23. What is mirage?
Ans: A mirage is an optical illusion which occurs on a very hot day due to the total internal reflection of light.
24. Why does a diamond sparkle?
Ans: Diamond sparkle due to the total internal reflection.
25. Which rays are used by the doctor to sterilize their instruments?
Ans: Gamma rays are used by the doctors to sterilize their instruments.
26. The refractive index of water is 1.33. If the apparent depth of the pond is 8 m, calculate its real depth. Complete the given ray diagram.
Solution: Here,
Given,
Refractive index of water (μ) = 1.33
Apparent depth = 8 m Real depth =?
We have,
Refractive index (μ) = Real depth/Apprant depth
i.e 1.33 = Real depth / 8
∴Real depth = 10.64 m
Hence, the real depth of pond is 10.64 m.
In the given figure, a totally reflecting prism is shown. A totally reflecting prism is that which has one of its angle equal to 90 degree and each of the remaining two angles equal to 45 degree here the phenomenon of total internal reflection is used.
27. Study the given diagram and answer the following questions.
What is the name of ∠LSA in such condition?
What effect is seen to the ray SR when ∠LSA is increased? Mention the name of that condition?
Write one application of the process obtained due to ∠LSA.
Αns: The name of ∠LSA in such condition is critical angle.
When the ∠LSA is increased beyond the critical angle, the ray of light SR reflects back to denser medium (water), instead of refraction. This phenomenon of light is termed as total internal reflection of light.
Optical fiber is constructed using the principle of total internal reflection, which is used in medicine in endoscopes.
28. What is a totally reflecting prism?
Ans: A totally reflecting prism is an isosceles right-angled glass prism in which the total internal reflection of light takes place.
29. Write two laws of refraction of light.
Ans: The law of refraction of light states that:
The incident ray, the refracted ray and the normal at the point of incidence all lie on the same plane.
The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for a given pair of media. This is also called Snell's law. Mathematically,
μ = (Sine of the angle of incidence)/(Sine of the angle of refraction)
i.e., (Sin i)/(Sin r) =(μ) constant
The constant (μ) is called the refractive index of a medium.
30. What is electromagnetic wave?
Ans: The wave which does not require material medium to propagate and which can even travel in a vacuum is called an electromagnetic wave.
31. What is an electromagnetic spectrum?
Ans: The classification of electromagnetic waves according to frequency or wavelength is known as the electromagnetic spectrum.
32. Which colour will have greater speed in vacuum?
Ans: In vacuum or air, the rays of all colours of light have the same speed.
33. How are microwaves used for cooking purposes?
Ans: The microwave oven produces the microwaves which are strongly absorbed by the molecules in food that are used to cook the foods.
34. Which rays are used for satellite communications?
Ans: Radio waves are used for satellite communication.
35. Write two advantages and two disadvantages from the ultraviolet light.
Ans: Advantages from the ultraviolet ray of light are as followings:
I. Ultraviolet radiation from the sun helps with the formation of vitamin D in the human body.
II. Ultraviolet rays are used in a burglar alarm, automatic door openers and counters and for detecting real pearl.
Disadvantages from the ultraviolet ray of light are as followings:
I. Excessive exposures to ultraviolet radiations are harmful to eyes and skin.
II. Ultra-violet radiation from the sun causes sun-tanning of the skin and may also lead to skin cancer.
36. Rays of short wavelengths of electromagnetic waves are dangerous to our body, why?
Ans: Rays of short wavelengths of electromagnetic waves are dangerous to our body, due to following reasons:
I. The rays of a short wavelength of electromagnetic waves such as x-rays, β-rays, γ-rays, α-rays, and ultraviolet rays can damage human DNA. They alter the structure of nitrogen bases or sugar-phosphate backbone in DNA molecule. They cause a mutation in human DNA.
II. X-Rays penetrate skin and muscle but not bones. If they interact with human DNA, they can cause cancer.
III. Infrared radiation is felt as heat and causes skin burns.
IV. Gamma rays are the highest frequency and most energetic form of light. They penetrate the human body and cause radiation poisoning after nuclear events like meltdowns.
37. Observe the diagram and answer the following questions.
Which phenomenon is shown in the diagram?
Which colour of the light has the shortest wavelength?
Write the name of an electromagnetic wave having the wavelength more than red.
Why does a ray of light split into seven different colour rays on passing through a prism?
Ans: The phenomenon shown in figure is dispersion of light by prism.
Violet colour of light has shortest wavelength.
The electromagnetic wave having the wavelength more than red is infrared ray.
White light is composed of seven different colour. In a vacuum (or air) all constituent colours (VIBGYOR) of white light have the same speed (i.e. 3 х 108). When white light enters a denser medium (prism), the speed of light decreases. The speed of the seven different colours of the white ray decreases at different rate. It shows that the red light has the maximum speed and the violet light has the minimum speed. Thus, the red light deviates the least and the violet light deviates the most. The deviation of other coloured rays such as blue, green etc. is also different because of their different velocities; this phenomenon is called dispersion of light. Due to dispersion of light white ray split into different colour while passing through prism.
38. Why does light bend on passing from one medium to the other?
Ans: Light bends on passing from one medium to the other due to the change in its velocity while entering from one optical medium to another.
39. What is meant by dispersion of light?
Ans: The phenomenon of splitting up of a white ray of light into its constituent seven colours is called dispersion of light