10.3 Check and Reflect
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Key Concept Review
1. The ray model of light uses straight lines to show how light travels
and interacts with matter.
2.
3. You can show the direction the light is moving by using lines with
arrows. You can show relative brightness of light by seeing if the
number of arrows changes from place to place.
4. Light may be reflected, absorbed, or transmitted.
5.
6. The nature of the surface of the object and the physical properties of
the material of the object determine the behaviour of incident light.
7. If light from a non-point light source shines on an opaque object, two
types of shadow form behind the object. The umbra is the area of
shadow where all rays of light from the source are blocked by the
object. The penumbra is the area of shadow where some light rays are
blocked by the object and some aren’t.
8. (a) In regular reflection, the reflected light all travels in the same
direction. In diffuse reflection, the reflected light is scattered in a
variety of directions.
(b) A mirror and shiny paint exhibit regular reflection; newsprint
paper and matte paint exhibit diffuse reflection.
9. The water is muddy and transmits a negligible amount of light. So the
water is (nearly) opaque. If any light did get through, there would be
so much scattering that no clear image could form—the water might
be barely translucent.
10.3 Check and Reflect
10.
Connect Your Understanding
11.
Light rays from around the gym reflect off the basketball. Because
the rays come from all directions and reflect diffusely in all directions
some light rays end up in my eye. I see the basketball with these
reflected rays.
12. Most metal is reasonably shiny while wood is usually dull. The shiny
metal would reflect light better. In addition, metal is easier to shape.
13.
If my hand is far from the light source, the light rays that graze my
fingers have to be quite close together and make a small shadow on
the wall. If my hand is close to the source, the light rays must be
spread out to go past my hand and a large shadow is formed.
Unit D: Light and Geometric Optics
55
10.3 Check and Reflect
14.Changing the surface of a material can affect its transparency; for
example, glass with an etched or frosted surface is translucent. A
change of state can cause a change in optical properties; for example,
transparent water may freeze into translucent or opaque ice. An
electric current may affect the transparency properties of liquid
crystals and many polymers.
15. A spotlight bulb is quite small, casts a reasonably narrow beam, and
is quite far from the actor. These three factors suggest that the actor’s
shadow should be quite sharp and dark (umbra).
16. Students’ answers may vary but might include: Certainly, no object is
perfectly transparent. Light is electromagnetic radiation and interacts
with the positive and negative charges in any material of the object as
it passes through. Some tiny bit of light will get absorbed or
scattered, ruining perfect transparency. Most opaque materials are
opaque for a certain range of electromagnetic radiation but
transparent for others. Glass is transparent for visible light but opaque
for ultraviolet. Cement is opaque for visible light but transparent for
radio waves and gamma rays.
17. Pigments easily absorb light and reflect only a small amount of a
particular colour. If white light shines on a thick pigmented object,
some coloured light reflects back from near the surface but all the rest
of the light is absorbed in the interior—the object appears opaque. If
a very thin slice of the same material is used, some coloured light
may be transmitted and the material will appear translucent.
18. Students’ answers may vary but should include similar main points:
Sunlight strikes the glass on top of the panel—some light will reflect
off and the rest will be transmitted. This light is now partially
absorbed by the water, heating the water. Since the panel is probably
not very thick, most of the light will be transmitted through the water
and be absorbed by the black surface of the metal, heating it. The
metal will either directly conduct heat to the water or emit infrared
radiation that will be absorbed by the water, heating it.
Reflection
19. Students’ answers may vary but could include: My vision is
extremely important and depends on how light interacts with the
tissues in my eye. Understanding how light behaves when it strikes
glass is important in designing energy efficient houses. Knowledge of
how to direct light and how to diffuse light is needed to build proper
lighting for indoor areas.
D: Light and Geometric Optics