CBSE Class 10 Science The Human Eye and the Colourful World MCQs Set 11

Answer: (b) real and inverted

Explanation:
1. The eye lens is a convex lens that forms a real, inverted and diminished image on the retina.
2. The brain interprets the signals so that we see objects upright.

Teacher's Note:
1. The retinal image is always real, inverted and smaller than the object.
2. The brain, not the eye, makes the picture appear erect.

Answer: (d) different amounts of light

Explanation:
1. The iris changes the size of the pupil.
2. This regulates the amount of light entering the eye; it does not do the focusing.

Teacher's Note:
1. Pupil size is linked to brightness, not distance.
2. Focusing for distance is done by the ciliary muscles.

Answer: (c) the eye is not able to decrease the focal length beyond a limit

Explanation:
1. For a very near object the focal length of the eye lens has to become very small.
2. The ciliary muscles can thicken the lens only up to a limit, so the focal length cannot be reduced beyond it and the image gets blurred.

Teacher's Note:
1. Eye strain while reading from very close is a sign of this limit.
2. The retina is only about 2.5 cm from the lens.

Answer: (d) A, B and C

Explanation:
1. Two eyes give a wider field of view than one.
2. Together they also give a three-dimensional view and help us judge height and distance, so A, B and C are all correct.

Teacher's Note:
1. Each eye sees a slightly different view and the brain combines them.
2. This is why one-eyed vision loses the sense of depth.

Answer: (a) Convex lens

Explanation:
1. He cannot see nearby objects, so he is suffering from hypermetropia.
2. Hypermetropia is corrected by a convex lens.

Teacher's Note:
1. Decide the defect from what the person cannot see.
2. Cannot see near means hypermetropia, which needs a convex lens.

Answer: (b) convex and concave lens

Explanation:
1. Hypermetropia is corrected by a converging (convex) lens.
2. Myopia is corrected by a diverging (concave) lens.

Teacher's Note:
1. Keep the order given in the question.
2. Hypermetropia first: convex. Myopia second: concave.

Answer: (a) the person is able to see clearly the distant objects, but not near objects

Explanation:
1. A far-sighted person sees distant objects clearly.
2. Nearby objects are blurred because their images form behind the retina.

Teacher's Note:
1. The name tells what you can see: far-sighted sees far.
2. Near-sighted sees near.

Answer: (c) hypermetropia

Explanation:
1. He cannot see nearby objects clearly, so his near point has moved from 25 cm to 75 cm.
2. This is hypermetropia; presbyopia is an old-age defect.

Teacher's Note:
1. Identify the defect from what the person cannot see.
2. Near objects blurred: hypermetropia.

Answer: (d) −15.8 cm, −6.3 D

Explanation:
1. The lens must form the image of the board (u = −300 cm) at the far point v = −15 cm.
2. 1/f = 1/v − 1/u = −1/15 + 1/300 = −19/300, so f ≈ −15.8 cm.
3. P = 100/f(cm) = −6.33 D, i.e. about −6.3 D.

Teacher's Note:
1. Use the lens formula with sign conventions: both u and v are negative here.
2. A negative focal length means a concave lens, as expected for a short-sighted student.

Answer: (c) A concave lens of 2.0 D

Explanation:
1. The lens must form the image of the poster (u = −2.0 m) at the person's far point v = −0.4 m.
2. 1/f = 1/v − 1/u = −2.5 + 0.5 = −2, so P = −2.0 D, a concave lens.

Teacher's Note:
1. The person is short-sighted, so a concave lens is needed.
2. Work in metres to get the power directly in dioptres.

Answer: (b) 2.5 cm

Explanation:
1. For an object at infinity the image forms on the retina, so f = v = 2.5 cm.
2. Hence the maximum focal length is 2.5 cm.

Teacher's Note:
1. Distant objects need the longest focal length of the eye lens.
2. The ciliary muscles are fully relaxed in this state.

Answer: (c) violet

Explanation:
1. In the visible spectrum wavelength increases from violet to red.
2. So violet has the shortest wavelength.

Teacher's Note:
1. Violet: shortest wavelength, highest frequency.
2. Red: longest wavelength, lowest frequency.

Answer: (c) the velocity of violet rays is smaller than that of red rays

Explanation:
1. In glass, red light travels fastest and violet light slowest.
2. Hence the velocity of violet is smaller than that of red.

Teacher's Note:
1. Slower speed means higher refractive index and more bending.
2. Violet is slowest, so it bends most.

Answer: (a) prism

Explanation:
1. A prism has two inclined refracting surfaces.
2. The angle between them is the angle of the prism.

Teacher's Note:
1. Angle of prism is a property of the prism itself.
2. The other angles depend on how the ray falls on it.

Answer: (d) Red, yellow, green, blue

Explanation:
1. Frequency increases as wavelength decreases.
2. The order of wavelength is red > yellow > green > blue, so frequency increases in the order red, yellow, green, blue.

Teacher's Note:
1. VIBGYOR runs from highest to lowest frequency.
2. Red has the lowest frequency, violet the highest.

Answer: (a) VIBGYOR

Explanation:
1. The seven colours of the spectrum from the most-bent to the least-bent are violet, indigo, blue, green, yellow, orange, red.
2. The initial letters give the word VIBGYOR.

Teacher's Note:
1. Learn VIBGYOR forwards and backwards.
2. From a prism the usual display has red at one end and violet at the other.

Answer: (b) the refracting surfaces are inclined at an angle

Explanation:
1. The laws of refraction hold at each face.
2. Because the two refracting faces are inclined to each other, the two bendings add up and the emergent ray is not parallel to the incident ray.

Teacher's Note:
1. In a slab the faces are parallel, so the ray emerges parallel.
2. In a prism the faces are inclined, so the ray is deviated.

Answer: (c) refracting surfaces

Explanation:
1. In a triangular prism, light enters and leaves through two rectangular faces.
2. These are the refracting surfaces of the prism.

Teacher's Note:
1. The triangular ends are not used for the path of light.
2. The edge where the refracting surfaces meet is the refracting edge.

Answer: (c) Refraction, dispersion and internal reflection

Explanation:
1. Sunlight is refracted and dispersed on entering a raindrop.
2. It then undergoes internal reflection inside the drop and is refracted again as it leaves, giving the rainbow.

Teacher's Note:
1. A raindrop acts like a small prism with an internal reflection.
2. Scattering is not involved.

Answer: (c) refraction of light

Explanation:
1. Starlight bends towards the normal as it enters denser layers of the atmosphere.
2. The star therefore appears slightly higher than its actual position because of atmospheric refraction.

Teacher's Note:
1. Apparent shift of a star: refraction.
2. Colour effects such as the blue sky are due to scattering.

Answer: (c) atmospheric refraction

Explanation:
1. Near the horizon the rays from the top and bottom of the Sun's disc are refracted by different amounts by the atmosphere.
2. This makes the Sun look flattened or oval.

Teacher's Note:
1. Flattened Sun is another effect of atmospheric refraction.
2. Its red colour is due to scattering.

Answer: (a) dim

Explanation:
1. Twinkling is a change in the amount of starlight reaching the eye.
2. When less starlight is refracted towards us, the star appears dim; when more is refracted towards us it appears bright.

Teacher's Note:
1. Brightness depends on how much light reaches the eye.
2. The flickering between bright and dim is twinkling.

Answer: (a) light is least scattered

Explanation:
1. At noon the Sun is overhead and its light travels the shortest path through the atmosphere.
2. Very little scattering occurs, so the Sun appears white.

Teacher's Note:
1. Short path, little scattering, white Sun.
2. Long path at sunrise and sunset gives a red Sun.

Answer: (c) Red

Explanation:
1. Scattering is strongest for short wavelengths and weakest for long wavelengths.
2. Red has the longest wavelength among the options, so it is scattered the least.

Teacher's Note:
1. Remember: long wavelength, least scattering.
2. This is why red is used for danger signals.

Answer: (c) P and Q

Explanation:
1. Small particles scatter the short wavelengths such as blue and violet; larger particles scatter the longer wavelengths.
2. Particles P (350 nm) and Q (430 nm) are the smallest, so they scatter the shorter wavelengths.

Teacher's Note:
1. Particle size and wavelength go together.
2. Smaller particle, shorter wavelength scattered.

Answer: (d) A is false, but R is true.

Explanation:
1. Hypermetropia is far-sightedness, so A is false.
2. In myopia the image of a distant object forms in front of the retina, so R is a true statement.

Teacher's Note:
1. Near-sightedness is myopia.
2. Check whether R is true on its own even if it does not match A.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. Refractive defects can indeed be corrected with contact lenses or surgery, so A is true.
2. The eye lens is made of fibrous jelly-like material, so R is true, but it does not explain why correction is possible.

Teacher's Note:
1. The source's key said R explains A, but its own explanation said it does not.
2. Material of the lens is not the reason defects can be corrected.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. The near point of the eye is about 25 cm because the ciliary muscles cannot shorten the focal length beyond a limit.
2. Closer than that the image is blurred and the muscles strain, so R explains A.

Teacher's Note:
1. R gives the reason for A.
2. Hold reading material at about 25 cm.

Answer: (d) A is false, but R is true.

Explanation:
1. The splitting of white light into colours is dispersion; the band of colours is the spectrum. So A is false.
2. R is a true statement about scattering.

Teacher's Note:
1. Dispersion is the splitting, spectrum is the band.
2. Check definitions word by word.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. Light from a star is a single point source, and its apparent brightness varies as the atmosphere refracts it by a changing amount.
2. A planet is an extended source made of many points; the variations average out, so it does not twinkle. R explains A.

Teacher's Note:
1. The source's R said stars are smaller than planets, which is not the right reason.
2. Distance and extended size decide twinkling.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. Between the Sun and the earth is mostly vacuum, where all colours travel at the same speed, so there is no dispersion and sunlight stays white.
2. R states exactly why, so R explains A.

Teacher's Note:
1. Dispersion needs a medium in which speed depends on colour.
2. Vacuum is non-dispersive.

Answer: (c) A is true, but R is false.

Explanation:
1. At noon the Sun is overhead and its light travels the shortest path, so little scattering occurs and the Sun looks white; A is true.
2. R says the path is longer, which is false.

Teacher's Note:
1. At sunrise and sunset the path is longer.
2. Short path at noon, little scattering.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. A rainbow is formed by dispersion of sunlight in raindrops, so A is true.
2. Shorter wavelengths do scatter more, so R is true, but it is not the explanation of a rainbow.

Teacher's Note:
1. Rainbow is dispersion; blue sky is scattering.
2. Do not link them as cause and effect.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. The greater the refractive index, the more the light bends.
2. Violet has the highest refractive index in glass and bends the most, so R explains A.

Teacher's Note:
1. The source printed A the wrong way round (violet least, red most).
2. Violet bends most, red least.