Question

Experiments are done to test the optical properties of lenses immersed in media having different indices of refraction.

Experiment 1: A lens made of flint glass index of refraction 1.720 is tested. A beam of parallel light rays is sent into the lens, and the distance from the lens to the point of convergence of the beam is measured. This is the focal length of the lens. This focal length is measured with the lens immersed in media of various indices of refraction.

Medium	Index of refraction	Focal length (cm)
Air	1	8
Folinol	1.24	13
Water	1.33	20
11% Sugar solution	1.5	39
Carbon disulfide	1.62	95

*Rays do not converge at all.

Experiment 2: Another lens is tested. It is made of the same kind of glass as in Experiment 1. but this lens is thicker, more strongly curved.

Medium	Index of refraction	Focal length (cm)
Air	1	5
Folinol	1.24	8
Water	1.33	12
11% Sugar solution	1.5	24
Carbon disulfide	1.62	60
Methylene iodide	1.74	*

Experiment 3: A lens made of new plastic is tested. This lens is identical in size and shape to the glass in Experiment 2.

Medium	Index of refraction	Focal length (cm)
Air	1	13
Folinol	1.24	34
Water	1.33	360
11% Sugar solution	1.5	*
Carbon disulfide	1.62	*
Methylene iodide	1.74	*

The index of refraction of the plastic lens in Experiment 3 must be-

• A. less than 1.33
• B. between 1.33 and 1.50
• C. more than 1.33
• D. more than 1.50

Answer 1

The correct option is B

between 1.33 and 1.50

Explanation for correct option(s):

Option B:

1. When the refractive index of the lens is equal to the medium then the lens behaves as a rectangular glass so we can not get any kind of focus
2. As for plastic, we get the focus when the refractive index of the medium is 1.33 so the refractive index of the plastic is more than 1.33.
3. As for plastic, we could not get the focus when the refractive index of the medium is 1.50 so the refractive index of the plastic is less than 1.50.
4. By the explanation in points 2 and 3 we can say that the refractive index will be in the range of 1.33 to 1.50.

Explanation for incorrect option(s):

Option A:

1. As for plastic, we get the focus when the refractive index of the medium is 1.33 so the refractive index of the plastic is more than 1.33.

Option C:

1. As for plastic, we could not get the focus when the refractive index of the medium is 1.50 so the refractive index of the plastic is less than 1.50.

Option D:

1. As for plastic, we could not get the focus when the refractive index of the medium is 1.50 so the refractive index of the plastic is less than 1.50.

Final answer: Hence, option (B) is correct.