Class 10 – Science Extra Questions – Ch. 10 – The Human Eye and the Colourful World

Ans: C

Explanation: The near point of vision for a healthy young eye is typically around 25 centimeters. This is the closest distance at which the ciliary muscles can contract sufficiently to focus light from an object onto the retina. 25 meters and 2.5 meters are far too distant for the near point. 2.5 cm is much too close and would require extreme accommodation beyond normal capabilities.
Correct Option: C

Ans: A

Explanation: When a light ray enters a rectangular glass slab from air, it refracts towards the normal. This means the angle of refraction (∠r) will be less than the angle of incidence (∠i). When the light ray emerges from the glass slab back into the air, it refracts away from the normal. For a rectangular slab, the two refracting surfaces are parallel. Due to this parallelism, the angle of emergence (∠e) will be equal to the angle of incidence (∠i). Therefore, the angle of incidence (∠i) will be greater than the angle of refraction (∠r) but (nearly) equal to the angle of emergence (∠e).
Correct Option: A

Ans: C

Explanation: The sky appears blue due to a phenomenon called Rayleigh scattering. Sunlight, which is composed of all colors of the visible spectrum, enters the Earth’s atmosphere. The atmosphere contains tiny particles, primarily nitrogen and oxygen molecules. These particles are much smaller than the wavelengths of visible light. Rayleigh scattering states that shorter wavelengths of light (like blue and violet) are scattered more effectively by these small particles than longer wavelengths (like red and orange). While violet light is scattered even more than blue, our eyes are more sensitive to blue light, and some violet light is absorbed in the upper atmosphere, making the sky appear predominantly blue. Option A is incorrect because while some sunlight is absorbed, scattering is the primary reason for the blue color. Option B is incorrect because water vapor does scatter light, but it doesn’t scatter all colors equally, and the primary reason for the blue sky is not water vapor. Option D is incorrect as light bending (refraction) is not the primary cause of the blue sky; scattering is.
Correct Option: C

Ans: A

Explanation: A person with normal eyesight can see objects clearly at a wide range of distances. The near point of vision for a healthy eye is typically around 25 centimeters (or 0.25 meters). However, the far point of vision for a healthy eye is considered to be at infinity, meaning they can focus on very distant objects. Therefore, the range of clear vision extends from a very close distance (near point) to infinity (far point). Option A best represents this broad range, as “a few centimeters” covers the near point, and “infinity” covers the far point. Options B, C, and D provide limited ranges that do not encompass the full capability of normal vision.
Correct Option: A

Ans: C

Explanation: Myopia, or nearsightedness, occurs when the eyeball is too long or the cornea/lens is too curved. This causes light from distant objects to converge in front of the retina, rather than directly on it. As a result, the image formed on the retina is blurred. Option A describes hyperopia (farsightedness), and Option B describes a normal eye. Option D is incorrect as the lens is responsible for focusing light, but the focal point is formed at a specific location.
Correct Option: C

Ans: A

Explanation: Myopia, also known as nearsightedness, is a refractive error where the eye focuses images in front of the retina rather than on the retina. This results in clear vision for close objects but blurry vision for distant objects. Hyperopia (farsightedness) causes the opposite problem. Astigmatism is a condition where the cornea or lens has an irregular shape, leading to distorted vision at all distances. Presbyopia is age-related farsightedness that develops as the lens loses its flexibility.
Correct Option: A

Ans: B

Explanation: The question states that a person can clearly see objects at a distance of 50 cm and beyond. This means their near point (the closest distance at which an object can be seen clearly) is 50 cm. A normal human eye has a near point of approximately 25 cm. When the near point is further away than 25 cm, it indicates a condition where the eye cannot focus on nearby objects. This condition is known as hypermetropia (farsightedness). People with hypermetropia can see distant objects clearly but have difficulty seeing near objects. Myopia (nearsightedness) is the opposite, where distant objects appear blurred. Presbyopia is age-related loss of the eye’s ability to focus on near objects, and astigmatism is a condition where the eye does not focus light evenly onto the retina. Since the person can see clearly at 50 cm and beyond, their vision for distant objects is fine, but their ability to see objects closer than 50 cm is impaired, characteristic of hypermetropia.
Correct Option: B

Ans: B

Explanation: The prescription indicates negative diopter values (-3.00 and -3.50). Negative diopters are used to correct myopia, which is nearsightedness. Myopia is a refractive error where distant objects appear blurry because the eye focuses images in front of the retina instead of on it. Presbyopia is age-related farsightedness, astigmatism is a condition where the eye’s cornea or lens has an irregular shape, and hypermetropia (farsightedness) involves positive diopters for correction.
Correct Option: B

Ans: C

Explanation: The power of a lens is measured in diopters (D). A positive power indicates a converging lens, while a negative power indicates a diverging lens. A power of +2.0 D means the lens is converging. Converging lenses are used to correct hypermetropia (farsightedness) because they help focus light rays onto the retina when the eyeball is too short or the lens is not strong enough. Myopia (nearsightedness) is corrected by diverging lenses. Therefore, a converging lens with a power of +2.0 D corrects hypermetropia.
Correct Option: C

Ans: D

Explanation: Light travels at a finite speed. When we observe distant celestial objects, the light that reaches us has taken a considerable amount of time to travel from the object to our telescopes. Therefore, the image we see is not of the object as it is at the present moment, but rather how it was at the time the light left it. This is often referred to as looking back in time.
Correct Option: D