Chapter 14 Refraction

1. Chapter 14Refraction

2. 14.1 Refraction Refraction of Light • This blurred flower can be seen at the boundary of the water and the air around it. • The bending of light as it travels from one medium to another is called refraction. • If light travels from one transparent medium to another at an angle other than straight, the light rays change direction. • The angle between the refracted ray and the normal is called the angle of refraction.

3. Refraction occurs when light’s velocity changes • Glass, water, ice and diamonds are all examples of transparent media through which light can pass. • The speed of light in each is also different. • For example, the speed of light in water is less than air. The speed of light in glass is slower than in water. • When light moves at an angle from one medium to anotherthat slows it down, the ray is bent towards the normal. • From slower to faster, the opposite occurs. Refraction of Light

4. Refraction can be explained in terms of the wave model of light • The direction of the wave propagation is shown by the red arrow or light ray. • As the light enters the glass, the wave front slows down. • The slower wave fronts travel a smaller distance than do the wave fronts in the air. • This causes the entire plane wave to change directions. • The new waves are now traveling a shorter distance so their wavelengths will now become shorter as well. • However their frequency stays the same (v = ƒλ) Wave fronts

5. The Law of Refraction • Index of refraction is the ratio of the speed of light in a vacuum to the speed of light in that substance. • Light always travels fastest in a vacuum, so all values in table 1 will be greater than one. • The more the light is refracted, the higherthe index of refraction. • Air is very similar to a vacuum.

6. Objects appear to be in different positions due to refraction • Refraction causes the fish to appear closer than it really is to the cat. • Refraction is reversible so the fish will see the cat appear farther away. • The reason size appears to change is best explained by the dashed line in the diagram. The object’s image is moved in line with the path of the original ray. • Air has an index of refraction that is less than water, causing the ray to bend inward.

7. Wavelength affects the index of refraction • Pure light is made up of many wavelengths. • To get an exact index of refraction for the data in this table, an average wavelength of 589 nm must be used. • If we were to just use the wavelength of red light it would refract differently than say green light. • This best explained by considering how a prism separates the different wavelengths of light.

8. Snell’s law determines the angle of refraction • The index of refraction tells us how much a light ray will be bent as it passes from one medium to another. • The greater the index, the more the light will be bent. • In 1621, Willebrord Snell developed this formula to calculate refraction angles.

9. Practice ASnell’s Law • A light ray traveling through the air (ni = 1.00) strikes a smooth, flat slab of crown glass (nr= 1.52) at an angle of 30.00 to the normal. • Find the angle of refraction, Өr.   Answer 19.20

10. Questions1. The bending of light as it travels from one medium to another is called _________.2. T / F The speed of light in water is less than air. The speed of light in glass is slower than in water.3. Index of _________ is the ratio of the speed of light in a vacuum to the speed of light in that substance.4. Air has an index of refraction that is less than water, causing the ray to bend (inward / outward)? 5. T / F The greater the index of refraction, the less the light will be bent. refraction true refraction _____ false

11. 14.2 Thin Lenses Types of Lenses • When light enters a pane of glass at an angle, its velocity slows and is bent towards the normal. • When it exits, it speeds up again and bends away from the normal. • Light rays are bent as much entering the medium as they are exiting.

12. Curved surfaces change the direction of light • When the surfaces of a medium are curved, the direction of the normal line differs. • When light passes through these curved surfaces, the direction of the light rays varies from point to point. • A lens uses refraction to form images. • These images can be both real and virtual like with mirrors.

13. A real image is formed when the rays of light actually intersect to form the image. Real images can also be projected onto a screen. • Virtual images form at a point from which light rays appear to come but do not actually come. • Lenses are used to form images in optical instruments like cameras, telescopes and microscopes. • Below are examples of a converging lens and a diverging lens.

14. Focal length is the image distance for an infinite object distance • As with mirrors, lenses have a focal point. • With a converging lens, the focal point is where the image will focus (a). • Diverging lenses have their focal points on the same side as the rays originate. A dashed line is shown to line up the out going rays back to their focus (b). • The focal length is the distance from the center of the lens to the focus . Lenses

15. Ray diagrams of thin-lens system help identify image height and location • When the thickness of the lens is small compared to the radius of curvature, this is known as a thin-lens. • The front of the lens is the area where the light rays originate. • The back of the lens is where the rays have undergone refraction. • Remember, any rays that travel through the exact center of a lens undergo no refraction.

16. Characteristics of Lenses Converging lenses can produce real or virtual images of real objects • The following examples will show where images will form using converging lenses. • Keep in mind that all real images are able to be projected on a screen. • This example shows how an object at a very far distance can still be focused into one spot.

17. In this example our object is more than two focal lengths from the lens. • This causes a smaller image to form between F and 2F on the backside. • Examples include: camera, human eyeball and telescope.

18. When our object is right at 2F, our image is inverted and focused at the same distance on the other side.

19. When our object is just inside 2F, our image is magnified and forms just out side 2F on the back side. • Examples include; projectors and microscopes.

20. Projecting light from the focal point allows projection with parallel rays. • Lighthouses and search- lights use this design.

21. When the object is inside the focus, a magnified virtual image results. • This is one of few configurations that produces a virtual and upright image. • Examples include; magnifying glass and binoculars. Convex

22. Practice BConverging Lens • An object is placed 30.0 cm in front of a converging lens. • The focal length is 10.0 cm. • What will the image distance and magnification be? • Will the image be virtual or real?  Answer q = 15.0 cm, M = -0.500, real

23. Diverging lenses produce virtual images from real objects • A diverging lens creates a virtual image of a real object placed anywhere with respect to the lens. • The image is upright, and the magnification is always less than the object. • Additionally, the image always appears somewhere inside the focal point.

24. Use the following steps in drawing a ray diagram for a diverging lens. • First ray; is drawn parallel to the axis off the top of the object, then extends outward from the focus on the back side of the lens. • Second ray; extends through the center of the lens and therefore is not refracted. • Third ray: is drawn as if it was going to the focal point on the back side of the lens, but then backs up to show the top of the image. Concave

25. The Thin-lens Equation and Magnification • The thin-lens equation can be applied to both the converging and diverging lenses if we use this table. • All images formed in front of the lens, virtual image, will have a negative distance. • A converging lens has a positive focal length and a diverging lens has a negative focal length.

26. Magnification by a lens depends on the object and image distances • For magnification of our image, we will use the same formula as we did for spherical mirrors. • If we have a negative sign, this indicates our image is real and inverted. • A positive sign signifies the image is upright and virtual.

27. Practice BDiverging Lens • An object is placed 12.5 cm in front of a diverging lens. • The focal length is -10.0 cm. • What will the image distance and magnification be? • Will the image be virtual or real? Answer q = -5.56 cm, M = 0.445, virtual

28. Eyeglasses and Contact Lenses • The human eye is like a camera. It has a main lens called the cornea. • The cornea directs rays of light to the back of the eye called the retina. • In some people, the light is focused behind the retina. • This condition is called hyperopia, but is better known as farsighted and can be corrected with eyeglasses.

29. In some people the light is focused in front of the retina. • This is known as myopia or nearsightedness. • It can be corrected with eyeglasses that use a diverging lens. • Contact lenses work the same as eyeglasses. • The difference is that water under the contact is also part of the refracting medium of the contact.

30. Combination of Thin Lenses • If two lenses are used to magnify an image we use the following… • First, we calculate where the image should be using the first lens. • Where the image forms from the first lens, we use that location as an object distance for the second lens. • The second lens forms the final image.

31. Compound microscopes use two converging lenses • Greater magnification can be achieved by combining two lenses in a device called a compound microscope. • It consists of two lenses, an objective lenses, near the object, and an eyepiece. • The eyepiece serves as a simple magnifier that enlarges the already magnified objective image. This produces a real, inverted image. Compound lenses

33. Refracting telescopes also use two converging lenses • In a refracting telescope, an image is formed at the eye similar to that of a microscope. • Because the object is at infinity, the image forms at the focal point of the objective lens, Fo. • With our object still at infinity, the focal point of the eyepiece is located near the focal point of the object lens. • The focal point of the eyepiece, Fc , allows the eyepiece to act as a simple magnifier. Telescope

34. Questions1. When light enters a pane of glass at an angle, its velocity (slows / speeds up) and is bent towards the normal. When it exits, it speeds up and bends away from the normal.2. T / F When light passes through curved surfaces, the direction of the light rays varies from point to point.3. With a converging lens, the focal point is where the image will ______4. In diverging lenses, the image always appears somewhere inside the focal point and is (larger / smaller)?5. In some people, light is focused in front of the retina,this is known as myopia or ______________ _____ true focus ______ nearsightedness

35. 14.3 Optical Phenomena Total Internal Reflection • Internal reflection can occur when light travels from a medium of higher refraction index to that of a lower one. • At a particular angle of incidence, called the critical angle, the refracted ray may exceed 90 degrees. • For angles greater than the critical angle, the ray is entirely reflected at the boundary. Video

36. Because the sine of 90o equals 1, the following relationship results… • Note, this equation can only be used when ni is greater than nr. • In other words, total internal reflection occurs when light travels along a path from a medium of higher index of refraction to a lower one. • Critical angle is small for substances with large indexes of refraction like diamonds, n = 2.419

37. Practice CCritical Angle • Find the critical angle for a water-air boundary if the index of refraction of water is 1.333. ni= 1.333 nr= 1.000  Answer 48.6o

39. Atmospheric Refraction • Refraction within our atmosphere allows us to see sunsets even after the sun is below the horizon. • Our atmosphere refracts the sun’s light rays like a lens.

40. Refracted light produces mirages • A mirage can be observed when the ground is so hot that the air directly above it is warmer than the air higher up. • These layers of air at different heights have different densities and different refractive indexes. • The rays traveling in the cooler air move straight. • The light rays traveling through the hotter air are refracted and cause an inverted image. • The observer sees what looks like the reflection of the tree in a lake, indicating there is water.

41. Dispersion • Snell’s law indicates that incoming light of different wavelengths is bent at different angles as it moves into a refracting material. • This is known as dispersion. • For example, red light (λ = 650 nm) will refract the least, due to its longer wavelength, while blue light (λ = 470 nm) will refract the most.

42. White light passed through a prism produces a visible spectrum • Because of dispersion, the blue component of this incoming ray is bent more than the red. • As light emerges from the prism, the colors fan out in what we call the visible spectrum.

43. Rainbows are created by dispersion of light in water droplets • The dispersion of light into a spectrum is demonstrated in nature by a rainbow. • Light is refracted as it enters the drops. Violet is refracted the most and red the least. • Then these colors are reflected off the back of the drop and refracted again as they exit. • The violet ray makes a 40o angle and the red a 42o angle. Rainbow

44. Lens Aberrations • In the last chapter we learned that light rays that are far from the principle axis become out of focus. • This spherical aberration occurs as well with lenses. The light rays do not all converge at the focus. • Another condition with lenses is that each color wavelength of light will focus differently. • This is known as chromatic aberration. Cameras use a combination of converging and diverging lenses to help reduce this effect.

45. Questions1. At a particular angle of incidence, called the ______ angle, the refracted ray may be reflected at the boundary.2. A _______ can be observed when the ground is so hot that the air at the surface has a different density and refractive index then cooler air above.3. As light emerges from a prism, the colors fan out in what we call the visible _________.4. For a ________ to form, violet light is refracted the most as it enters the drops, and the color red the least.5. To reduce _________ aberration, cameras use a combination of converging and diverging lenses. critical mirage spectrum rainbow chromatic

46. End