1. A plano-convex lens is one with a convex surface on one side and a flat surfa

Question

1. A plano-convex lens is one with a convex surface on one side and a flat surface on the other. Suppose a thin lens is made of glass with index of refraction 1.5. If the radius of curvature of the curved surface is 1.1 meters, what is the focal length of the lens? Give your answer in meters.

2. A lens with two convex surfaces, both with radii of curvature of 0.25 meters, is used to focus light. If the lens is made of crown glass, it would have an index of refraction for red (656 nm) light of 1.509 and for blue light (486 nm) of 1.517. What happens when it is used to make an image of a distant source of white light?

Pick those that apply.

1. The blue light focuses closer to the lens than the red light.

2. When the red light is in focus it is surrounded by a blue halo.

3. The red light focuses closer to the lens than the blue light.

4. The red light falls in a cone outside the blue light as it the light converges to a focus, so when blue is in focus there is a fuzzy red ring around it.

1. The blue light focuses closer to the lens than the red light.

2. When the red light is in focus it is surrounded by a blue halo.

3. The red light focuses closer to the lens than the blue light.

4. The red light falls in a cone outside the blue light as it the light converges to a focus, so when blue is in focus there is a fuzzy red ring around it.

Explanation / Answer

from lens maker's formula

1/f = (n-1)*(1/R1 - 1/R2)

R1 = radius of curvature of convex surface = 1.1 m

R2= radius of curvature of planesurface = infinity

n = refractive index of material = 1.5

1/f = (1.5-1)*(1/1.1 - 1/infinity)

focal length f = 2.2 m

===========================

2)

for red light

R1 = 0.25 m

R2 = -0.25 m

n = 1.509

1/f1 = (1.509-1)*(1/0.25 + 1/0.25)

f1 = 0.245 m

for blue light

R1 = 0.25 m

R2 = -0.25 m

n = 1.517

1/f2 = (1.517-1)*(1/0.25 + 1/0.25)

f2 = 0.242 m

The blue light focuses closer to the lens than the red light.

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