lesson 2: reflection and mirrors (page 322) key questions: – 1. what are the kinds of reflection?...

Lesson 2: Reflection and Mirrors (page 322)

• Key Questions:– 1. What are the Kinds of Reflection?– 2. What types of Images Do Mirrors Produce?

• Sunshine Standards: – SC.7.P.10.2: Observe and explain that light can

be reflected, refracted, and/or absorbed.

What are the kinds of reflection?

• Do you know why you can see your reflection in a mirror and not in your textbook?

• To understand this you need to understand how a surface reflects lights.


• To show how light reflects, you can represent light waves as straight lines called rays.

• You may already know that light obeys the law of reflection: the angle of reflection equals the angle of incidence.


• The two waves in which a surface can reflect light are regular reflection and diffuse reflection.

Regular Reflection

• This occurs when parallel rays of light hit a smooth surface.

• All the light rays reflect at the same angle because of the smooth surface. So you can see a clear image.

Regular Reflection

• An Image is a copy of the object formed by reflected or refracted light rays.

• Shiny surfaces such as metal, glass, or calm water produce regular images.

Diffuse Reflection• This type of reflection

occurs when parallel rays of light hit an uneven surface.

• Each light ray obeys the law of reflection, but hits the surface at a different angle because the surface is uneven. Therefore, each ray reflects at a different angle.

Diffuse Reflection

• You either don’t see an image, or the image is not clear.

• Most objects reflect light diffusely. This is because most surfaces are not smooth.

What types of images do mirrors produce?

• Plane mirror: a flat sheet of glass that has a smooth silver colored coating on one side.

• Often, the coating is on the back, and protects the mirror from damage. The light hits the coating and regular reflection occurs, this occurs because the coating is smooth.

Virtual Images• The image you see in a plane mirror is a virtual

images.

• This is an image that forms where light seems to come from.

• “Virtual” describes something that does not really exist. Your image appears to be behind the mirror, but you cannot reach behind the mirror and touch it.

Plane Mirror

• A plane mirror produces a virtual image that is upright and the same size as the object.

• But the image is not quite the same as the object. The left and right of the image are reversed.

Concave Mirror

• A mirror with a surface that curves inward like the inside of a bowl.

• A concave mirror can reflect parallel rays of light so that theymeet at a point. These rays are

alsoparallel to the optical axis.

Concave Mirror

• The optical axis is an imaginary line that divides a mirror in half.

• The point at which the rays parallel to the optical axis reflect and meet is called the focal point.

Concave Mirror

• The type of image formed by a concave mirror depends on the location of the object.

• Concave mirrors can produce real or virtual images.

• A real image form when light rays actually meet.

Real Images in Concave Mirrors• If the object is farther away from the mirror than

the focal point, the reflected rays form a real image.

• Unlike a virtual image, a real image can be projected on a surface such as a piece of paper. Real images are upside down.

• A real image may be smaller, larger, or the same size as the object.

Virtual Images is Concave Mirrors

• If an object is between the mirror and the focal point, the reflected rays form a virtual image.

• Virtual images formed by a concave mirror are always larger than the object.

• Concave mirrors produce the magnified images you see in a makeup mirror.

Convex Mirror

• A mirror with a surface that curves outward is called the convex mirror.

• The reflected rays spread out but appear to come from a focal point behind the mirror.

• The focal point of a convex mirror is the point from which the rays appear to come.

Convex Mirror• A convex mirror

produces a virtual image that is always smaller than the object.

• These are the types of mirrors used in “rear-view” mirrors on cars. The advantage to this type of mirror is that it allows you to see a larger area than you can with a plane mirror.

Convex Mirror

• The disadvantage is that the image is reduced in size, and therefore appears farther away than it actually is.

Lesson 3: Refraction and Lens• Key Questions:– 1. What happens when light hits an object?– 2. What determines the type of image formed by a

lens?– 3. What factors affect the speed of a wave?

• Sunshine Standards:– SC.7.P.10.2: Observe and explain that light can be reflected, refracted, and/or

absorbed.– SC.7.N.1.6: Explain that empirical evidence is the cumulative body of observations

of a natural phenomenon on which scientific explanations are based. – SC.7.N.1.7: Explain that scientific knowledge is the result of a great deal of debate

and confirmation within the scientific community. – SC.7.N.2.1: Identify an instance from the history of science in which scientific

knowledge has changed when new evidence or new interpretations are encountered.

What happens when light hits an object?

• An object can absorb some or all of the light that hits it. An object can also bend the light that passes through it, creating double images or rainbow.

• When light hits an object, it can be reflected , refracted, and/or absorbed.

What happens when light hits an object?

• The more transparent an object is, the less light it will absorb. An opaque object will both reflect and absorb light.

• The light may also refract or reflect.

Refraction

• Refraction can cause you to see something that may not actually be there.

• Example of The Fish Tank: A fish tank can play tricks on your eyes. If you look through the side of the fish tank, the fish seems closer than if you look at it from the top. If you look through the corner of the tank, you may see the same fish twice. How does this happen?

Refraction• As you look at the fish through the tank, the light

coming from the fish to your eye bends as it passes through three different mediums. The mediums are water, the glass of the tank, and air.

• As the light passes through the mediums, the light is refracted.

• When the light rays enter a new medium at an

angle, the change in speed causes the rays to bend.

Reflection in Different Mediums• Some mediums bend light more than others.

When light passes from one medium into another, the light slows down.

• Light slows down again and bends even more when it passes from water to glass.

• When light passes from glass back into air, it speeds up.

Reflection in Different Mediums• Notice from the next

figure, that the ray that leaves the glass is traveling in the same direction as it was before it entered the water.

• Glass causes light to bend more than either air or water. Another way to say this is that glass has a higher index of refraction than either air or water.

Refraction in Different Mediums

• The index of refraction of a medium is a measure of how much a light ray bends when it enters the medium.

• The higher the index of refraction of a medium, the more it bends light.

Prisms and Rainbow• Recall that when white light enters a prism, each wavelength is

refracted by a different amount. The longer the wavelength, the less the wave is bent by a prism.

• Red, with the longest wavelength, is refracted the least. Violet, with the shortest wavelength, is refracted the most.

Prisms and Rainbows

• The difference in refraction causes the white light to spread out into the colors of the spectrum. The same process occurs in water droplets.

• When the sun shines through the droplets suspended in the air, a rainbow may appear. The water droplets act like tiny prisms. Refracting and reflecting the light and separating colors.

• A mirage is an image of distant object caused by refraction of light.

Mirage

Mirage• In this image you can see what appears to be the a reflection of the truck

in the road. The air just above the road is hotter than the air higher up. • Light travels faster in hot air, so light rays from the truck that travel

toward the road are bent upward by the hot air. Your brain however, assumes they travel in a straight line. The rays look like they have reflected off a smooth surface, but really you are seeing a mirage.

What determines the type of image formed by a lens?

• A lens is a curved piece of glass or other transparent material that refracts light.

• A lens forms an image by refracting light rays that pass through it. Lenses can have different shapes.

• The type of image formed by a lens depends on the shape and position of the object.

Concave Lens

• A concave lens is thinner in the center than at the edges.

• When light rays traveling parallel to the optical axis pass through a concave lens, they bend away from the optical axis and never meet.

• A concave lens always produces a virtual image that is upright and smaller than the object.

Concave Lens

• This figure shows how a concave lens forms an image. The image is located where the light rays appear to come from.

Convex Lens• A convex lens is thicker in the center than at the

edges.

• As light rays parallel to the optical axis pass through a convex lens, they are bent towards the center of the lens. The rays meet at the focal point of the lens and continue to travel beyond.

• The more curved the lens is, the more it refracts light. A convex lens acts like a concave mirror, because it focuses rays of light.

Convex Lens

• An object’s position relative to the focus point determines whether a convex lens forms a real or virtual image.

Convex Lens• When an object is between the lens and the

focal point, the refracted rays form a virtual image. The image forms on the same side of the lens as the object, and is larger than the object (like a magnifying glass on a book.)

Convex Lens• If the object is outside of the focal point, the

refracted rays form a real image on the other side of the lens. The real image can be smaller, larger, or the same size as the object.

What Factors Affect the Speed of a Wave?

• Waves move at different speeds through different mediums.

• The speed of a wave through a substance is determined by the substance’s physical properties.

• Light, like all electromagnetic waves, consists of vibrating electric and magnetic fields. The speed of light through a medium depends on how the medium interacts with electric and magnetic fields.

What factors affect the speed of a wave?

• Some substances will cause the light to move at a slower speed that it will through other substances.

• If the speed of light is the same in two substances, light will not refract when it passes between these substances.

Factors that affect the speed of a wave…

Sound Waves

• Sound waves also travel at different speeds through different substances.

• Temperature is one factor that can affect the speed of sound.

Sound Waves and Temperature• The speed of sound in air is about 344 m/s at

room temperature. As temperature increases, the speed of sound in air also increases. For solids, however, temperature increase causes the wave’s speed to decrease.

Sound Waves

• Two factors that determine the speed of sound through a substance are the compressibility and density of that substance.

• If either the compressibility or the density of a substance increases, the speed of sound through that substance will decrease. When comparing the speeds of sound through two different substances, both compressibility and density must be considered.

Seismic Waves

• The same thing happens with seismic waves. They travel under the Earth’s surface. Their speed is affected by the density and compressibility of the material they pass through.