D'Amato PTHS 20071

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

D'Amato PTHS 20072

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.)

D'Amato PTHS 20073

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

This is NOT the way it works

How does the shadow of the ball appear?

How does this support the correct idea?

Each point sends light rays in every direction

D'Amato PTHS 20074

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?

D'Amato PTHS 20075

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.

D'Amato PTHS 20076

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

O

A

BC

D

D'Amato PTHS 20077

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 Smooth surface, i.e. M

irror

A

O

B

C

IncidentBeam

ReflectedBeam

D'Amato PTHS 20078

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.

Mirror

Incident rays

D'Amato PTHS 20079

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.

Ordinary surface

Incident rays

D'Amato PTHS 200710

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.

air something else air

D'Amato PTHS 200711

Total internal reflection and the critical angle

At a boundary between two substances, light will reflect rather than refracting if the incident angle is greater than the critical angle.

This only happens when the incident medium has a higher n than the refracting medium

air 1.0n

something else

1n

D'Amato PTHS 200712

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

D'Amato PTHS 200713

If Tim is standing outside in the bright sun, why is his shadow not completely black?

1 2 3 4 5

0 0 000

1. Light rays bend around Tim

2. The sun is larger than Tim

3. The grass in Tim's shadow holds light from the sun

4. Other objects are reflecting light

5. The sun is the only luminous object in this situation

D'Amato PTHS 200714

Which term does not belong with the others?

1 2 3 4 5

0 0 000

1. Perpendicular

2. Tangent

3. Normal

4. At right angles

5. 90°

D'Amato PTHS 200715

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 θ1 of the beam on the water?

1 2 3 4 5

0 0 000

1. 60°

2. 90°

3. 30°

4. 0°

5. There is not enough information to say

D'Amato PTHS 200716

What is the most likely path of the light ray below after it has passed entirely through the glass prism?(The dotted line is a radius)

1 2 3 4 5 6

0 0 0000

1. bent upward

2. bent downward

3. following its original path

4. a new path parallel to its original path

5. something else

6. there is not enough information to tell

D'Amato PTHS 200717

What is the most likely path of thislight ray after it has passed entirelythrough this glass prism?

1 2 3 4 5 6

0 0 0000

1. The ray will be bent upward

2. The ray will be bent downward

3. The ray will be following its original path

4. The ray will be on a new path parallel to its original path

5. The ray will go in some other direction

6. There is not enough information to tell

D'Amato PTHS 200718

Which term does not belong?

1 2 3 4 5 6

0 0 0000

1. Material

2. Substance

3. Medium

4. Index of refraction

5. Gas

6. Liquid

D'Amato PTHS 200719

Which term does not belong?

1 2 3 4 5

0 0 000

1. Density

2. Color

3. Index of refraction

4. Amount

5. Hardness

D'Amato PTHS 200720

Which of these statements is true about a surface that exhibits specular reflection?

1 2 3 4 5

0 0 000

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

D'Amato PTHS 200721

Which of these statements is true about a surface that exhibits diffuse reflection?

1 2 3 4 5

0 0 000

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

D'Amato PTHS 200722

Which is a good explanation for why you can’t normally see a laser beam from the side in midair?

1 2 3 4 5

0 0 000

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

D'Amato PTHS 200723

Which of these phenomena is NOT predicted by the idea that a luminous object emits light rays from each point in all directions?

1 2 3 4

0 000

1. Fuzzy shadows

2. A spoon looks bent in a glass of water

3. Pinhole projector

4. Camera obscura