Review Notes For Optics

How we see

• Vision happens when a ray of light enters an eye • Without a source of light, there can be no vision at all • Rays of light travel in straight lines

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Luminous and illuminated objects

• Objects that create light are called luminous objects. Examples include the sun, a candle, a light bulb • Other objects do not create light but instead are illuminated by other light sources • (Draw a ray diagram showing how a candle allows you to see your hand in a dark room.)

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Each point on an object emits or reflects light rays in every direction

This is NOT the way it works

Each point sends light rays in every direction

How does the shadow of the ball appear? How does this support the correct idea? 3

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An experiment that shows how each point sends light rays in all directions

What do you see when you perform the experiment below? Why does this happen?

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Remember the basics about angles

• You need a corner between two lines to have an angle • An angle is measured at the corner where two lines meet • When two lines form an angle of 90° they are said to be perpendicular or normal to each other.

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Circle facts

If point O is the center and point A is on the circle: • Line CD is tangent to the circle at point A • Line OB is normal to the circle at point A

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C

B A

O

D

Reflection from a smooth surface

• AO is the incident beam • OB is the reflected beam • OC is the normal, an imaginary line drawn 90° from the surface at the point of reflection Law of reflection: • ∠AOC = ∠BOC • Angle of incidence = angle of reflection 7

B

Reflected Beam

C

Sm oo

th

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su rfa ce O ,i .e. M

A Incident Beam irr or

Reflection from a smooth surface

If a surface is smooth at the microscopic level, like a mirror, the normal lines at different locations are parallel to each other. Thus, parallel incident rays are reflected at the same angle. This type of reflection is called specular reflection. 8

Incident rays

Mirror

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Reflection from an irregular surface

Ordinary surfaces are rough and bumpy at the microscopic level, and the normal lines point in all different directions. Thus, parallel incident rays will be reflected in all directions. A beam of light will be reflected diffusely in all directions, so this is called diffuse reflection.

Incident rays

Ordinary surface

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Refraction

When light travels from one medium into another medium (like going from air into water) its path changes when it crosses the surface. This is called refraction. A normal line is imagined, perpendicular to the surface between the two media, at the point the light ray crosses. Going from air into something else, the ray crosses the normal line and bends towards the normal in the other medium. Going from something else into air, the ray crosses the normal line again and bends away from the normal. 10

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air

something else air

Seeing an image

The direction of the light ray that reaches your eye determines where you see the image In this example, two observers would agree that the image of the candle flame is behind the mirror

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Reflection from a concave mirror

• Reflections from a curved mirror also obey the law of reflection • The normal to a circular shape always goes through the center O of the circle

incident beam

Norm al

O reflected beam 12

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Definition of the focal point

Light rays that are incident parallel to the main axis will be reflected through the mirror's focal point The focal point is halfway between the center O and the surface of the mirror

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focal point

O center

main axis

Rays through the focal point

Reflection works the same way in reverse: Rays that pass through the focal point will be reflected parallel to the main axis focal point

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O center

main axis

Other easy rays to reflect from a concave mirror

Any ray that passes through the center will be reflected straight back • Can you explain why? O center

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Locating an image on a concave mirror

To find the image of a small object, draw some easy rays from the object and see how they are reflected The image is located at the place where observers would see the reflected rays diverging from

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small object

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O

f image location

Which is NOT a condition required for you to see object X?

1. A light ray must reach your eye from object X 2. Object X must emit light rays 3. There must be an uninterrupted straight line between your eye and object X 4. All of the above are required for you to see object X 5. More than one of the above are NOT required

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The best answer is 2. You can also see an object that only reflects light rays

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If Tim is standing outside in the bright sun, why is his shadow not completely black? 1. Light rays bend around The best answer is 4. Light Tim rays are reflected from other objects and 2. The sun is larger than Tim illuminate the area in 3. The grass in Tim's Tim's shadow shadow holds light from the sun 4. Other objects are reflecting light 5. The sun is the only luminous object in this situation 18

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Which term does not belong with the others?

1. 2. 3. 4. 5.

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Perpendicular Tangent Normal At right angles 90°

The best answer is 2. All the other words are about 90° angles

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A laser beam is aimed at the top of a tank of water. The beam makes an angle of 60° with the surface of the water. What is the incident angle of the beam on the water?

1. 2. 3. 4. 5.

60° 90° 30° 0° There is not enough information to say

Make a sketch and remember the definition of "angle of incidence". The answer is 3. beam

normal

30° 60° water

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Which of these statements is TRUE about a surface that exhibits specular reflection? 1. All reflected light rays are parallel, no matter where they come from 2. Normal lines are parallel at all points on the surface 3. The light ray bends toward the normal when it reflects 4. Normal lines point in different directions from the surface 5. More than one of these is true

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The best answer is 2.

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Which of these statements is TRUE about a flat surface that exhibits DIFFUSE reflection? 1. Parallel incident rays will produce parallel reflected rays 2. An incident beam of light will be reflected in all directions 3. The incident angle of each ray is not equal to the reflected angle 4. Even if the surface looks rough to the naked eye, it is perfectly smooth at the microscopic level 5. More than one of these is true

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Both 2 and 4 and correct, so 5 is the best choice

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Which is a good explanation for why you can’t normally see a laser beam from the side in midair? 1. A laser beam is made of many light rays, and light rays always go straight 2. You need light in order to see, but there is no light in the middle of the beam 3. A laser beam is a special kind of light that goes in only one direction 4. If there was chalk dust in the air, you could see the laser beam 5. None of these is a good explanation 23

Some of these are true statements, but none of them are good explanations of why you can't see the beam in midair.

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