Light and ReflectionSection 1 © Houghton Mifflin Harcourt Publishing Company Preview Section 1...

Light and Reflection Section 1

© Houghton Mifflin Harcourt Publishing Company

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Section 1 Characteristics of Light

Section 2 Flat Mirrors

Section 3 Curved Mirrors

Section 4 Color and Polarization



The student is expected to:TEKS

7B investigate and analyze characteristics of waves,

including velocity, frequency, amplitude, and

wavelength, and calculate using the relationship

between wavespeed, frequency, and wavelength;

7C compare characteristics and behaviors of

transverse waves, including electromagnetic waves

and the electromagnetic spectrum, and

characteristics and behaviors of longitudinal waves,

including sound waves



What do you think?

• What are electromagnetic waves?• Are there different types of electromagnetic

waves? • If so, what are they?• How are they similar?• How are they different?

• Do all electromagnetic waves travel at the same speed?• If so, what is it?



Electromagnetic (EM) Waves

• Visible light can be separated into a spectrum.– Red through violet

• Visible light is very small part of a larger spectrum, the electromagnetic wave spectrum.

• All EM waves travel at the same speed, the speed of light (c).– In a vacuum, v = c = 3.00 108 m/s (186,000

miles/s).



• Electromagnetic waves consist of electric and magnetic fields.– The fields are mutually

perpendicular.

Electromagnetic (EM) Waves



Click below to watch the Visual Concept.

Visual Concept

Electromagnetic Waves





Classroom Practice Problems

• The middle of the visible spectrum is green light. Calculate the wavelength for green light if the frequency is 5.5 x 1014 Hz.– Answer: 5.4 x 10-7 m or 0.00054 mm

• The middle of the audible spectrum is 1.0 x 104 Hz (10 000 Hz). Calculate the wavelength of this sound in air if the temperature is 25°C.– Answer: 0.035 m or 35 mm

• By what factor are the sound waves farther apart than the light waves?– 65 000 times



Waves and Rays

• Huygen’s principle states that each point on a wave front acts as a source for new waves.

• The diagram shows five points on the initial front sending out waves. These waves are part of the new front.



Illumination• Illumination is measured in lumens (lm).• Illumination depends on the brightness of the source

and the distance from the source.

– Predict the relationship between illumination and distance.• It is an inverse square relationship.



Now what do you think?

• What are electromagnetic waves?• Are there different types of electromagnetic

waves? • If so, what are they?• How are they similar?• How are they different?• Do all electromagnetic waves travel at the same

speed?• If so, what is it?



The student is expected to:TEKS

7D investigate behaviors of waves, including

reflection, refraction, diffraction, interference,

resonance, and the Doppler effect

7E describe and predict image formation

as a consequence of reflection from a

plane mirror and refraction through a thin convex

lens



What do you think?

• We use our depth perception to determine the distance to an object. When you look in a flat mirror, you see your image.

• How far away does the image appear?• Is the image at the mirror, farther away than the

mirror, or closer than the mirror?

• Make a sketch showing the following: • a side view of yourself as a stick figure, the mirror,

and the reflected light that creates the image



What do you think?

• Imagine a small flat mirror mounted on a vertical wall. You are about a meter away. Because the mirror is small, you see only from your nose to your belt buckle. Now you start backing away from the mirror.

• As you back up, will you see more of your body, less of your body, or the same nose to belt image?• Why do you think so? (Explain your answer through

personal experiences or a sketch.)



Reflection

• Diffuse reflection is reflection from a rough surface, such as notebook paper or the wall.

• Specular reflection is reflection from a smooth surface, such as a mirror or shiny metal.– We will study specular reflection.



Angle of Reflection

• At what angle will the incoming ray reflect from the mirror?

• Angle of incidence () = Angle of reflection (’)

• By convention, angles are measured with a normal line.– Angles with the surface are

also equal.



Image in a Flat Mirror - Ray Diagram

• Ray 1 strikes the mirror and reflects at the same angle (90°).

• Ray 2 reflects at the same angle it strikes.

• Our eyes see the two reflected rays and many others.

• The brain assumes that they came from a common point, and the image is seen at that point.



• The image is behind the mirror.

• The image distance (q) equals the object distance (p).

• The image size (h’) equals the object size (h).

• The image is virtual, not real.– Reflected rays do not actually

meet, they only appear to come from a common point.

Image in a Flat Mirror - Ray Diagram




Visual Concept

Comparing Real and Virtual Images



Flat Mirrors

• Imagine a small mirror as shown. The man will only see the portion of his body shown. How would that change if he were closer to the mirror? Farther away?– Try drawing a

diagram similar to that shown.




• We use our depth perception to determine the distance to an object. When you look in a flat mirror, you see your image.

• How far away does the image appear?– Is the image at the mirror, farther away than the

mirror, or closer than the mirror?

• Make a sketch showing the following: – a side view of yourself as a stick figure, the mirror,

and the reflected light that creates the image




• Imagine a small flat mirror mounted on a vertical wall. You are about a meter away. Because the mirror is small, you see only from your nose to your belt buckle. Now you start backing away from the mirror.

• As you back up, will you see more of your body, less of your body, or the same nose to belt image?– Why do you think so? (Explain your answer through

personal experiences or a sketch.)



What do you think?

• Suppose a concave makeup mirror is held against a vertical wall while you look at yourself from the the opposite side of the room.• Would your image appear the same as it would in a

plane mirror? If not, how is it different?• How would this image change in size and appearance

as you approached the mirror? • How would the image appear if you were only a foot

(30 cm) from the mirror?



Concave Spherical Mirrors

• Mirrors that are a small portion of the inside of a sphere• The angle of incidence still equals the angle of reflection.• Called converging mirrors• The focal length (f) is one-half the radius (R).



Real Images

• The image shown is real because the reflected rays actually pass through each other.– Virtual images only appear

to come from a single point.

– Object distance (p)– Image distance (q)– Object height (h)– Image height (h’)



Ray Diagrams - Rules

• These rules describe three rays that are easily drawn without the need to measure angles.– Others can be drawn after the image point is located using at

least two of these rays.




Visual Concept

Rules for Drawing Reference Rays for Mirrors



Ray Diagram • Is the image real or virtual? Inverted or upright? Larger than or smaller than or equal to the object in size?– Try drawing the ray

diagram locating the image of the pencil if the object is placed at C.

– Now try it for the object between C and F.

– See the next slide for drawings.



• Is each image real or virtual? Inverted or upright? Larger than or smaller than or equal to the object in size?

– Now try drawing the ray diagram locating the image of the pencil if the object is placed at F.

– Finally, try it for an object beyond F (closer to the mirror).– See the next slide for drawings.



• Is each image real or virtual? Inverted or upright? Larger than or smaller than or equal to the object in size?




Visual Concept

Ray Tracing for a Concave Spherical Mirror



Mirror Equation

• p and q have a positive value if they are on the front side or reflecting side of the mirror.– Real images

• q has a negative value if the image is on the back side of the mirror.– Virtual image

• f is positive for concave mirrors.



Equation for Magnification

• h is positive if it is upright and negative when inverted.• M is positive for virtual (upright) images.



Classroom Practice Problem

• When an object is placed 30.0 cm in front of a concave mirror, a real image is formed 60.0 cm from the mirror’s surface. Find the focal length.– Answer: 20.0 cm




• A square object is placed 15 cm in front of a concave mirror with a focal length of 25 cm. A round object is placed 45 cm in front of the same mirror. Find the image distance, magnification, and type of image formed for each object. Draw a ray diagram for each.– Answers: Square - virtual image, q = -38 cm, M = 2.5– Answers: Round - real image, q = 56 cm, M = -1.2– Ray diagrams should look similar to (4) and (6) in

Table 4.



Convex Mirrors

• Called a diverging mirror because rays are spread out by the mirror

• Image is always virtual and smaller than the object.



Convex Mirrors

• Used as side-view mirrors on cars– What warning is written on these mirrors? Why?– Images are small so they appear to be farther away.

• Also used in stores to monitor shoppers• Equations are the same as those for concave mirrors.




Visual Concept

Ray Tracing for a Convex Spherical Mirror




• A convex mirror has a radius of curvature of 12.0 cm. Where is the focal point?– Answer: f = -6.00 cm (behind the mirror)

• Find the position of the image for an object placed the following distances from the mirror: 50.0 cm, 30.0 cm, 12.0 cm and 2.00 cm.– Answers: -5.35 cm, -5.00 cm, -4.00 cm, -1.50 cm



Parabolic Mirrors & Spherical Aberration

• With spherical mirrors, rays not near the principal axis do not all meet at the image point.

• Parabolic mirrors eliminate this problem and produce sharper images.



Parabolic Mirrors & Spherical Aberration

• Using rays near the axis on spherical mirrors reduces the aberration or blurriness of the image.– A very small section of a sphere is nearly identical to

a paraboloid.

• Parabolic mirrors are used in telescopes to sharpen the image.




Visual Concept

Reflecting Telescope




• Suppose a concave makeup mirror is held against a vertical wall while you look at yourself from the the opposite side of the room.– Would your image appear the same as it would in a

plane mirror? If not, how is it different?– How would this image change in size and appearance

as you approached the mirror? – How would the image appear if you were only a foot

(30 cm) from the mirror?



What do you think?

• Imagine a darkened room with two projectors shining light on a screen. One shines blue light while the other shines yellow light. • What color will be seen when these

two colors overlap each other on the screen?

• Why do you think this is the case?• What experiences have you had that

helped you decide on the color?



What do you think?

• Suppose the colors are switched to red and green.• What color will be seen when

these two colors overlap each other on the screen?

• Why do you think this is the case?• What experiences have you

had that helped you decide on the color?



Color

• Why does a leaf appear green?

• Why do parts of the U.S. flag appear red?

• Objects appear a certain color because they absorb the other colors of the spectrum.– The leaf absorbs all but

green (see diagram).– The flag absorbs all but

red.



Color Addition

• White light is a mixture of all of the colors of the spectrum (ROYGBV).

• The primary additive colors are red, green, and blue.

• Addition of these colors with differing intensities produces all other colors.– TVs use closely spaced red, green, and blue pixels.



Color Addition

• Red + green ---> yellow• Red + blue ---> magenta• Blue + green ---> cyan

• Any two colors forming white are said to be complimentary colors.– Yellow and blue– Magenta and green– Cyan and red




Visual Concept

Additive Color Mixing



Color Subtraction and Pigments

• Another way to form different colors is by subtraction.– Pigments and dyes absorb (or subtract) some colors

and reflect (or transmit) others.– Leaves subtract red and blue but reflect green.

• The primary pigments for color subtraction are cyan, magenta, and yellow.– Color printers use CYM cartridges.

• These have three colors of ink and mix them to produce all other colors.



Color Subtraction and Pigments• Yellow is a combination of red and green. A

yellow pigment reflects both red and green or it removes blue.– In other words, yellow pigments subtract blue light.– Similarly, cyan pigments subtract red light.

• Therefore, if you mix yellow and cyan pigments, blue and red are both subtracted, and you see green reflected.– Use subtraction to determine the color seen if you mix

• cyan and magenta • yellow and cyan



Color Subtraction and Pigments

• Mixing all three pigments produces black.

• Different quantities of cyan, magenta, and yellow can produce the “millions” of colors possible on printers.




Visual Concept

Subtractive Color Mixing



Polarization of Light

• Unpolarized light consists of light with the electric and magnetic fields vibrating in all directions.

• Polarized light waves have fields vibrating in only one plane.– In this case, the electric field

is vertically polarized.



Polarizing Light

• Polarizing filters only allow light with the electric field aligned with the transmission axis to pass through.



Naturally Polarized Light

• Light reflected off a shiny surface like water is polarized by the reflective process.– Which way should the transmission axis be oriented to block out

“glare” light?– Why do fishermen like polarized sunglasses?



Naturally Polarized Light

• The light scattered off particles in the atmosphere is also polarized.

• Photographers use polarized filters to darken the blue sky and make clouds stand out.




Visual Concept

Polarization by Reflecting and Scattering




• Imagine a darkened room with two projectors shining light on a screen. One shines blue light while the other shines yellow light. • What color will be seen when these

two colors overlap each other on the screen?

• What color would be produced if they were red and green? Blue and green? Red and blue? Magenta and green? Cyan and red? Cyan and yellow?

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Light and ReflectionSection 1 © Houghton Mifflin Harcourt Publishing Company Preview Section 1...

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