Refraction of light pg. 77

Objectives Physics terms Identify refraction in everyday life. refracted ray angle of refraction index of refraction normal incident ray angle of incidence Focal length Converging lens Diverging lens Magnification Identify refraction in everyday life. Describe the relationship between the angle of refraction and the index of refraction. Predict the location and properties of an image using the thin lens formula. Calculate the magnification of an image using the object and image distance.

Refraction Why does this straw look broken? When light crosses a boundary between materials, it may change direction through refraction. Light reflecting off the straw above the water goes straight to your eye. Light reflecting off the straw below the water bends—refracts—as it passes into the air, so it appears to have come from a different direction.

Reflection and refraction Light may reflect at the boundary between two materials, staying in the original medium. Light may refract as it crosses a boundary between two materials, changing its direction. When light hits glass, it can do both.

Reflection and refraction In this window you can see items inside the store AND the woman’s reflection. This is because the light reflects and refracts at the same time. When you look out a window at night, you can see your reflection in the glass. Since the light also refracts, someone standing outside can see you too.

Refraction reminders Refraction is a property of all waves. Refraction occurs at a boundary between two materials. Light refracts because it travels at different speeds in different materials.

Visualizing refraction The normal is a line perpendicular to the boundary at the point where the ray strikes the new medium. The angle of incidence lies between the incident ray and the normal. The angle of refraction lies between the refracted ray and the normal. Angle of incidence Angle of refraction

The direction of refraction When light slows down it bends towards the normal. When light speeds up it bends away from the normal. Point out that the path of the light ray is reversible.

The amount of refraction The amount of refraction depends on the combination of materials. Which combination of materials results in greater refraction? air to glass air to water Notice the greater deflection of the refracted ray in the glass. Air to glass produces greater refraction. The incident ray has a greater deflection.

Test your knowledge This diagram shows four labeled angles. Which one of these is the angle of refraction? How do you know? θ2 θ1 θ4: It is the angle between the refracted ray and the normal. θ3 θ4 Mixing up the angles is a common source of student error.

The index of refraction Every light medium has an index of refraction (n) that determines how much it will refract light. The value of n is never less than 1, and has no units. Can you think of a material that might have a higher index of refraction than glass? Diamond’s high index of refraction is what gives it its brilliance and “fire”. There are materials with an even higher index of refraction than diamond, such as silicon crystals. Diamond has n = 2.42

The index of refraction The amount of refraction depends on the difference between the two indices of refraction. The bigger the difference, the greater the refraction. Which combination of air, water, and glass produces the least refraction? Ask the students which combination produces the greatest bending. glass and water produces the least bending.

The index of refraction Light passing from low to high index of refraction bends towards the normal. Light passing from high to low index of refraction bends away from the normal.

The amount of refraction Material 1 When light passes from air into water, the ray bends towards the normal. When light passes from air into glass, the ray bends even more towards the normal— because glass has a higher n. Material 2

Lenses A lens is an object used to refract light in specific ways.

Focal point Continuously curved surfaces allow a lens to refract the rays to a focal point. A light ray that comes in on the optical axis does not refract.

Focal point Continuously curved surfaces allow a lens to refract the rays to a focal point. A light ray that comes in on the optical axis does not refract. Light rays farther from the optical axis are refracted more.

Focal point The focal length is the distance from the center of the lens to the focal point. Highly curved lenses have short focal lengths. Slightly curved lenses have long focal lengths.

Types of lenses There are two kinds of lenses, converging and diverging. They differ in their shape and how they focus light.

Converging lens Converging lenses are convex: they are thicker in the middle and thinner at the edges. Converging lenses bend parallel light rays toward the optical axis. A magnifying glass is an example of a converging lens.

Diverging lens Diverging lenses are concave: They are thinner in the middle and thicker at the edges. Diverging lenses bend parallel light rays away from the optical axis. The rays appear to come from the near focal point Used for peepholes

Thin lens formula The thin lens formula relates the focal length of a lens to the object and image distances. If two of these properties are known, the third can be calculated using this formula. Example An object is placed 0.30 m from a lens with a focal length of 0.50 m. Where is the image located?

Thin Lens Example

Thin lens practice Don has a lens with a focal length of 60cm. If he places an object 80cm from the lens, where does the resulting image appear? 2. Felicia has an object 50cm from a lens. It creates an image 2.0m from the lens. What is the focal length of her lens?

Thin lens formula Click this calculator on page 613

Thin lens formula Observations How does the lens look when the focal length is 2.0m? When the focal length is changed to 20cm, does the focal length get closer or further from the lens? 3. When is the lens thicker, when the focal length is 2.0m or 20cm? 4. As the object distance increases, what happens to the image distance?

Magnification The thin lens formula can tell you where an image will be, and whether it is real or virtual. But what about the size of the image?

Magnification Magnification tells us the size of an image relative to its object. If m > 1, the image is enlarged. If m < 1, the image is reduced. If m is +, the image is upright. If m is -, the image is inverted.

Magnification Practice An object is placed 2.0m from a convex lens produces a real image located 80cm from the lens. What is the magnification of the image? An object is located 60cm away from a convex lens with a focal length of 40cm. What is the resulting magnification? An image that is 1.0 x 10-2 m tall is formed on a screen behind a converging lens when an object 2.0m tall is placed 8.0m in front of the lens. What is the distance from the lens to the screen? When a 4.0m tall object is placed 6m in front of a convex lens, an image is formed on a screen located 0.050m behind the lens. What is the size of the image?

Homework Give an example of when refraction occurs at home. when you view a spoon in a glass of water when you use a magnifying glass or binoculars when you look through a glass paperweight or a diamond ring

Homework 2. If a light beam shines from glass (n = 1.5) into air (n = 1.003), does it deflect towards the normal or away from it? 3. If the glass is replaced with diamond (n = 2.4), does the beam deflect more or less? Glass has a higher index of refraction than air so the beam bends away from the normal. The difference in the index of refraction is greater so the beam deflects more.

Homework 4. On the diagram below, label the incident angle, incident ray, refracted angle, refracted ray, and the normal.

Homework 4. On the diagram below, label the incident angle, incident ray, refracted angle, refracted ray, and the normal.

Homework 5. Marco places an object 40 cm from a thin convex lens with a focal length of 20 cm. How far is the image from the lens? 10 cm 20 cm 30 cm 40 cm

Homework 6. Teresa places an object 50 cm away from a lens, and the resulting image appears at 80 cm. What is the magnification of the image? This image is inverted (-) and larger than the object.