8. The persistence of vision for human eye is
a) 1/10th of a second b) 1/16th of a second c) 1/6th of second d) 1/4th of second
9. Two convex lenses of focal length 10cm and 20cm are kept in contact. The focal
length of the combination is
a) 20/3cm b) 40cm c) 12cm d)15cm
10. When we enter a cinema hall, we cannot see properly for a short time. This is
because
a) Pupil does not open b) Pupil does not close c) Adjustment of size of pupil delays
d) none of these
11. When light falls at critical angle on the surface of a rarer medium while coming
from a denser medium, the refracting angle is
12. The power of accommodation for a normal eye is
13. A person suffering from both myopia and hypermetropia uses
lenses.
14. What is total internal reflection?
15. Why is a normal eve not able to see clearly the objects placed closer than 25 cm?
16. Define atmospheric refraction.
17. State the function of rods and cones in human eye.
Answers
Answer:
8. B) 1/16th of a second
9. A) 20/3 cm
Explanation:
Focal length of the lenses are 10 cm amd 20 cm
So the power of both the lenses will be 1/10 and 1/20
The power of the combination of both lenses = 1/10 + 1/20 = 3/20
And focal length is reciprocal of power
Hence, the answer is 20/3 cm
10. C) Adjustment of size of pupil delays
11. When light falls at critical angle on the surface of a rarer medium while coming from a denser medium, the refracting angle is 90°
12. The power of accomodation of a normal eye is 4D as the near point = 25cm (= 25/100 m)
focal length = 0.25
Therefore when we calculate
P = 1/f
==> Power = 1/0.25
==> P = 100/25 = 4 Dioptre.
13. Person uses Bifocal lenses in which upper part consists of a concave lens (for myopia) and the lower part consists of a convex lens (for hypermetropia).
14. When the refracted rays goes along the direction of interface of two medium or along the boundary of separation of two medium and the angle of incidence foer which the angle of refraction becomes 90° is called Total Internal Refraction (TIR).
15. A normal eye is unable to clearly see the objects placed closer than 25 cm because the ciliary muscles of eyes are unable to contract beyond a certain limit.
16. The refraction of light caused by earth's atmosphere (having layers of varying optical density) is called Atmospheric Refraction.
17. Rods provide vision during dim light and Cons provide vision at bright light.