CBSE Class 10 Physics The Human Eye and the Colourful World Worksheet Set 08

Printable Worksheet Set 08 for CBSE Class 10 Physics Chapter 10, The Human Eye and the Colourful World, has twelve true or false statements with explanations and four numerical problems on the power of spectacle lenses for myopia and hypermetropia. It is a short sheet that prints well, covering the rainbow, prism, defects of vision, the colours of the Sun and clouds, and atmospheric refraction, and each answer is given below the question.

True or False Statements

Question 1. State whether true or false: The lens which is used for correcting the presbyopia defect of the eye is concave.
Answer:
False. Presbyopia is corrected by a convex lens (or a bifocal lens when myopia is also present).
In simple words: Presbyopia needs a convex or bifocal lens, not a concave lens.

Teacher's Note:
1. Concave lenses correct myopia.
2. Bifocal lenses have both.

Question 2. State whether true or false: The colour that deviates maximum while passing through a glass prism is violet.
Answer:
True. Violet has the largest refractive index in glass, so it is bent the most.
In simple words: Violet bends the most and red the least.

Teacher's Note:
1. Order of deviation: violet is the maximum.
2. Red is the minimum.

Question 3. State whether true or false: Water droplets act as tiny prisms in the formation of a rainbow.
Answer:
True. Sunlight is refracted, dispersed and internally reflected by the droplets, and the colours come out in different directions.
In simple words: Raindrops split sunlight like small prisms.

Teacher's Note:
1. Droplets must be present in the air.
2. The Sun must be behind the observer.

Question 4. State whether true or false: The transparent spherical membrane covering the front of the eye is known as the cornea.
Answer:
True. The cornea is the transparent front part of the eye through which light enters and where most of the refraction takes place.
In simple words: The cornea is the transparent window at the front.

Teacher's Note:
1. It does most of the bending of light.
2. The lens fine-tunes the focus.

Question 5. State whether true or false: The eye which can see nearby objects clearly is said to suffer from hypermetropia.
Answer:
False. An eye that can see nearby objects clearly but not distant ones has myopia. Hypermetropia is the defect in which nearby objects cannot be seen clearly.
In simple words: Seeing near clearly means myopia, not hypermetropia.

Teacher's Note:
1. Myopia: near is clear.
2. Hypermetropia: far is clear.

Question 6. State whether true or false: The eye which cannot see distant objects clearly is said to suffer from myopia.
Answer:
True. In myopia the image of a distant object forms in front of the retina, so distant objects look blurred.
In simple words: Not seeing far clearly is myopia.

Teacher's Note:
1. It is also called short-sightedness.
2. A concave lens corrects it.

Question 7. State whether true or false: In myopia the image of distant objects is focused before the retina.
Answer:
True. Because the eyeball is too long or the eye lens too curved, the rays converge in front of the retina.
In simple words: Myopic eyes focus distant objects in front of the retina.

Teacher's Note:
1. Concave lens moves the image back.
2. Link the position of the image to the defect.

Question 8. State whether true or false: Hypermetropia is corrected by using a convex lens of suitable power.
Answer:
True. A convex lens converges the rays, shifting the image of a nearby object from behind the retina on to the retina.
In simple words: A convex lens fixes far-sightedness.

Teacher's Note:
1. Convex lens: converging.
2. The power depends on the near point.

Question 9. State whether true or false: A person suffering from myopia cannot see distant objects clearly.
Answer:
True. Myopia is short-sightedness: nearby objects are seen clearly but distant ones are blurred.
In simple words: Myopic people see near but not far.

Teacher's Note:
1. The far point is less than infinity.
2. Correct with a concave lens.

Question 10. State whether true or false: The Sun looks red at sunset because most of the blue light in the sunrays is scattered, leaving behind red and yellow light.
Answer:
True. At sunset the light travels a long path in air, where the blue light is scattered away and the red reaches the observer.
In simple words: The long path removes the blue and leaves the red.

Teacher's Note:
1. Red has the longest wavelength.
2. At noon the Sun looks white.

Question 11. State whether true or false: Clouds look white because water droplets of clouds scatter all colours of light equally.
Answer:
True. The droplets are large compared with the wavelength of light, so they scatter all colours almost equally and the scattered light is white.
In simple words: Large drops scatter all colours, giving white clouds.

Teacher's Note:
1. Compare with small air molecules.
2. Size of the scatterer matters.

Question 12. State whether true or false: The Sun is visible about two minutes before the actual sunrise due to atmospheric refraction.
Answer:
True. Atmospheric refraction bends the light from the Sun when it is just below the horizon, so we see it a little earlier.
In simple words: Bent sunlight lets us see the Sun early.

Teacher's Note:
1. The same effect delays sunset by about 2 minutes.
2. Total gain is about 4 minutes.

Numerical Problems

Question 13. A student uses spectacles of focal length −60 cm. Name the defect of vision he is suffering from. Which lens is used for the correction of this defect? Compute the power of this lens.
Answer:
The lens has a negative focal length, so it is a concave (diverging) lens and the student has myopia.
f = −60 cm = −0.6 m.
P = 1/f = 1/(−0.6) = −1.67 D.
In simple words: A concave lens of −1.67 D corrects the student's myopia.

Teacher's Note:
1. Convert cm to m.
2. Concave lens: negative power.

Question 14. A person suffering from short-sightedness can see clearly only up to a distance of 2 metres. Find the nature and power of the lens required to correct his vision.
Answer:
The person has myopia, so he needs a concave lens.
f = −2 m, so P = 1/f = −0.5 D.
In simple words: A −0.5 D concave lens is required.

Teacher's Note:
1. Far point = 2 m = focal length magnitude.
2. Negative sign for concave lens.

Question 15. The near point of a person suffering from hypermetropia is 75 cm. Calculate the focal length and power of the lens required to enable him to read a newspaper which is kept at 25 cm from the eye.
Answer:
u = −25 cm and v = −75 cm. 1/f = 1/v − 1/u = −1/75 + 1/25 = 2/75.
f = 75/2 = +37.5 cm (convex lens).
P = 1/0.375 = +2.67 D .
In simple words: A +2.67 D convex lens lets him read at 25 cm.

Teacher's Note:
1. Use v for the defective near point.
2. Positive power: convex lens.

Question 16. The least distance of distinct vision of a long-sighted person is 40 cm. He wishes to reduce it to 25 cm by using spectacles. Find the power and nature of the lens used by him.
Answer:
u = −25 cm and v = −40 cm. 1/f = 1/v − 1/u = −1/40 + 1/25 = 3/200, so f = 200/3 = 66.7 cm.
P = 1/0.667 = +1.5 D, and the lens is convex.
In simple words: A +1.5 D convex lens reduces his near point to 25 cm.

Teacher's Note:
1. The image must be at the defective near point.
2. Check the arithmetic in fractions.