Uses of LensesOctober 30, 2014

Objectives

1. Diagram and define reflection and virtual image

2. Give examples of refraction and describe how it works

3. Diagram and describe the different types/uses of lenses

Law of Reflection

• The angle of reflection equals the angle of incidence

• Angle of Incidence – angle of strike (i)

• Angle of Reflection –angle of rebounds (r)

• Normal – imaginary line that extends perpendicular to the flat surface

Virtual Images

• Reflected image that we see in a mirror

• our minds perceive it not as a reflection, but as coming out from the mirror

Look at a mirror it, appears that you are behind that mirror in exactly the same position

Curved mirrors• Convex mirror – curves outward, virtual image is

smaller and closer

• Concave mirror – curves inward, virtual image is larger and further away

• What evidence can you use to support the claim that the frequency of light does not change upon reflection?

• If you took a picture of your image while standing 2 m in front of a plane mirror, what distance should you set your camera to provide the sharpest focus?

Refraction

• The bending of light due to changing speed, passing from one medium to another

• When light slows down (goes from gas to liquid) it refracts toward the normal

• When light speeds up (solid to air) it refracts away from the normal

• Why is it more dangerous to drive a car of a rainy day?

• If the speed of light were the same in all media, would refraction still occur when light passes from one medium to another?

Types of Lenses• concave lens – where light that passes through

diverges as if it came from a point

• Convex lens – where light that passes through converges to a single point

What are Lenses Used for?

1. Camera’s

2. Telescopes

3. Eyeglasses/Contacts

Camera’s

• Pinhole Camera:

• Simplest of camera’s

• A box with a pinhole in the front and film paper attached to the back

• Hole prevents overlapping rays and prints a very dim upside down image

• Improved lens Camera

• Lens admits more light (takes less time)

• Allows more rays without overlapping (brighter more focused image)

Explain in your own words how this works.

The Eye• How do the lenses in your eyes help you see?

Eyesight

• Nearsighted – can’t see far away

• Farsighted – can’t see close up

• What is the importance of the rays meeting at the retina?

Nearsighted Vision

• The lens becomes shorter and fatter

• Where do the rays come into focus?

• How can this be corrected?

Farsighted Vision

• The lens becomes longer and skinnier

• Where do the rays come into focus?

• How can this be corrected?

How are camera’s and our eyes similar

Telescopes

• Galileo

• Kepler

Galileo’s Telescope

Kepler’s Telescope

• Using a second concave lens magnified the image even more