Forming Images with Lenses

Question 1

Describe what happens to light in refraction. What affects focal length?

Answer 1

When light hits a boundary some is reflected with the rest transmitted. At an angle, the light is refracted as it changes speed. Refraction amount depends on the wavelength of the light. Focal length changes according to wavelength.

Question 2

How do lenses change the nature of travelling waves? Though speed across the wave changes, what doesn’t?

Answer 2

Lenses change curvature of wavefronts by refraction. Curvature is added, and waves which travel along the principle axis will curve with the middle facing the lens, as speed slows here. The waves will take the same time to reach the focal point. Ray diagrams show the waves as ray lines, focusing at the focal point after passing the lens axis.

Question 3

If a lens is more powerful, how are wavefronts affected? How does this affect the focal length?

Answer 3

The more powerful the lens, these are typically thicker, the more wavefronts are curved, reducing the focal length.

Question 4

What are two formulas for power/curvature?

Answer 4

P=1/f in Dioptres, D. Curvature=1/radius of curvature. Curvature is the power.