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	*

In Experiment 3, why do the rays not come to a focus at all when they pass through the lens into certain materials?

• A. The index of refraction of each of those materials is greater than that of the lens
• B. The curvature of the lens is not great enough
• C. Chemical reactions turn some of those materials opaque.
• D. In some materials, the light rays are unable to bend as they pass out of the lens.

Answer 1

The correct option is A

The index of refraction of each of those materials is greater than that of the lens

Explanation for correct option(s):

Option A:

1. If the refractive index of the medium is more than the refractive index of the lens then the convex lens behaves as a concave lens so we can not focus the light rays on any point.
2. If the refractive index of the medium is more than the refractive index of the lens then the convex lens behaves as a rectangular glass slab so we can not focus the light rays on any point.
3. Maybe the refractive index of the material for a lens used in experiment 3 is more than or equal to 1.5 so we are unable to find the focus of the lens.

Explanation for incorrect option(s):

Option B:

1. If the curvature of the lens is not great enough then we will not find the focus of the lens in any case.

Option C:

1. If the chemical reaction turns some of the materials opaque then the rays will not pass through the lens but in question, it is given that rays pass through the lens.

Option D:

1. The bending of light rays depends upon the refractive index of the lens with respect to the medium,

Final answer: Hence, Option (A) is correct.