Unit 2: Optics Part 2

Transcription

1 Unit 2: Optics Part 2

2 Refraction of Visible Light 1. Bent-stick effect: When light passes from one medium to another (for example, when a beam of light passes through air and into water, or vice versa), the change of speed causes it to change direction. This accounts for the well-known fact that a stick half under water looks bent: light from the submerged part of the stick changes direction as it reaches the surface, creating the illusion of the bent stick.

3 Refraction of Visible Light 2. Position of a fish under water: Fortunately for the fish, light refracts as it travels from the fish in the water to the eyes of the hunter. The refraction occurs at the water-air boundary. Due to this bending of the path of light, a fish appears to be at a location where it isn't. A visual distortion occurs. Subsequently, the hunter launches the spear at the location where the fish is thought to be and misses the fish. Some fish can overcome this to hunt for their prey!

4 Refraction of Visible Light Based on these two examples, what can you say about what you see when visible light is refracted?

5 Refraction We learned already that refraction is the bending of a wave, such as light, when it travels from one medium (material) to another. Waves bend because the speed of the waves change when travelling from one medium to another. When one end of the wave front slows down, the direction of the wave changes.

6 Using Ray Diagrams for Refraction As in reflection, we have 5 main components: 1. The incident ray: this is the incoming ray that will hit the solid surface/barrier (e.g. a mirror) 2. The refracted ray: the ray that describes the direction of light after passing from one medium to another of different density 3. The normal: a line perpendicular (at a 90 o angle) to the surface of the two media as it extends through both media 4. The angle of incidence: the angle between the incident ray and the normal 5. The angle of refraction: the angle between the normal and the refracted ray

7 Ray Diagrams for Refraction Label the following ray diagram using the terms on the previous slide.

8 How can the medium effect the speed of light? When light travels from one medium to another medium of greater density, it will slow down and refract towards the normal (for example, from air to water) When light travels from one medium to another medium of lower density, it will speed up and bend away from the normal.

9 Homework Page Page & 2 Page #s 3, 7, 8

10 Types of Lenses There are two types of lenses: 1. Convex lenses: cause light rays to bend toward each other, or converge. Convex lenses are thicker in the middle than at the edge. 2. Concave lenses: cause light rays to bend away from each other, or diverge. Concave lenses are thinner in the middle than at the edge.

11 Look at the diagrams on page 216 Think about what you already know about refraction as light rays travel from a less dense medium (e.g. air) towards a more dense medium (e.g. glass or plastic), they bend toward the normal. As the rays exit the other side of the lens, travelling from a more dense to a less dense medium, they bend away from the normal.

12 Examples of convex and concave lenses Convex lenses: Eyeglasses for farsightedness, magnifying glasses Concave lenses: Eyeglasses for nearsightedness ****Homework: Look on the internet and find two more examples of each type of lens****

13 Normal Vision When light rays from a distant object enter the eye, the rays are parallel. The lens, which is convex, causes the rays to converge at the retina, producing a sharp image. Light rays from a nearby object diverge when they enter the eye, so muscles in the eye cause the lens to change shape, making the lens thicker. This gives the lens a greater ability to converge the light rays to form a clear shape.

14 Near-sightedness Near-sighted people can see nearby objects clearly but cannot bring distant objects into focus (Ms. Saville). These people usually have to wear glasses/contact lenses all day long. This condition occurs because the lens converges the light rays to form an image in front of the retina. By the time the light rays actually strike the retina they have begun to spread out again, making the person see a fuzzy image. A concave lens is used to diverge the parallel rays slightly so that the image forms farther back, on the retina.

15 Far-Sightedness Far sighted people see distant objects clearly but find that nearby objects remain fuzzy no matter how hard they try to focus on them. In these people, an image falls behind the retina behind the eye has a shorter shape than a normal eye. These people usually wear glasses when reading. Light rays from distant objects are nearly parallel, and require less refraction to converge them than light from nearby objects. Light rays from nearby objects are diverging as they enter the eye. A convex lens is needed for the light rays to come into focus exactly on the retina.

16 Homework Page 231 #s 1-5 Page 236 #s 1-7 Page 241 #1-9

17 Double Convex Lenses Focal Point: the point at which parallel rays meet after passing through the lens. Focal length: the distance between the centre of the lens and the focal point. Each lens has its own focal length, which is determined by the curve of the lens. Lenses with greater curvatures will have shorter focal lengths.

18 What will it look like? Ray diagram Image Size Image distance Upright or Inverted? Real of Virtual? Object between lens and focal point Larger than the object Farther from the lens than the object Upright Virtual Object between focal point and 2X the focal length Larger than the object Farther from the lens than the object Inverted Real Object is more than twice as far from the lens as the focal point Smaller than the object Closer to the lens than the object Inverted Real

19 Double Concave Lenses To find the focal point in a double concave lens, draw the refracted rays, and then extend the rays backward as dotted lines. The point where the lines meet is the focal point. The focal length is still the distance between the centre of the lens and the focal point, and is still determined by the curvature of the lens. The lens with the greatest curvatures will have the shortest focal lengths.

20 What will it look like? Ray diagram Image Size Image distance Upright or Inverted? Real of Virtual? Object between lens and focal point Smaller than the object Smaller than the object distance Upright Virtual Object between focal point and 2X the focal length Smaller than the object. (Smaller than when the object is between the lens and focal point) Smaller than the object distance Upright Virtual Object is more than twice as far from the lens as the focal point Smaller than the object. (Smaller than when the object is between the focal point and 2X the focal length) Smaller than the object distance Upright Virtual

21 Homework Page 216 #s1-3 Page 221 #s 1-3 Page 223 #s 1-2 Page 227 #s 1-5, 7-10

22 Optical Technologies Until optical technologies were developed, our knowledge was limited to what we could see with our eyes. We can now use optical technologies to see the tiniest organism or far away in outer space. While the different types of optical technologies may seem very different, they are all based on the same understanding of light, mirrors and lenses. You can use ray diagrams to determine the image that different types of optical technologies produce, but the diagrams would be very complicated. We use simplified diagrams to explain how optical instruments work.

23 Microscopes A compound light microscope uses two convex lenses with relatively short focal lengths to magnify small, close objects. The object to be viewed is placed on a transparent slide and a light is shone from below. The light passes by or through the object on the slide and then travels through the objective lens.

24 Microscopes The objective lens is a convex lens. Recall that if the distance from an object to a convex lens is between one of the two focal points, it forms an enlarged image of the object. The eyepiece lens is another convex lens, magnifies the image again. The final image can be 100s of times larger than the actual object, depending on the focal lengths of the two lenses.

25 Telescopes The farther away an object, the more the amount of light reflecting off of that object decreases, so the object appears to be dimmer. Scientists have been developing telescopes for more than 400 years to gather and focus light from faraway stars. A telescope uses a lens or concave mirror that is much larger than your eye to gather more of the light from distant objects. The largest telescope can gather over a million times more light than the human eye. Because of this, distant galaxies appear much brighter. Because the image formed by a telescope is so much brighter, the image can be magnified to a greater extent to reveal more detail.

26 Refracting Telescopes Like a microscope, a telescope has an objective lens and an eyepiece. The objective lens in a telescope has a longer focal length than in the microscope because the objects viewed are far from the lens. The simplest telescopes have only two lenses. These lenses bend the light to focus it, which is why a telescope with this design is called a refracting telescope.

27 Refracting Telescopes The objective lens collects light and focuses it into an image. The image is formed inside the telescope and is never seen directly. Instead, the image is magnified by the eyepiece lens, and directed into the eye of the operator or into a camera. In order to get a detailed image of a distant object, the objective lens must be as large as possible. A large lens is heavy and can be supported in the telescope tube only around its edge. The lens can sag or flex due to its own weight, distorting the image it forms. Heavy glass lenses are costly and difficult to make, and even when the highest quality of glass is used, the lens absorbs some of the light.

28 Reflecting Telescopes Because of the problems with making large lenses, most large telescopes today are reflecting telescopes. A reflecting telescope uses a concave mirror, a plane mirror and a convex lens to collect and focus light from distant objects. Light from a distant object enters one end of the telescope and strikes a concave mirror at the opposite end. The light reflects off of this mirror and converges.

29 Reflecting Telescopes Before it converges at a focal point, the light strikes a plane mirror that is placed at an angle within the telescope tube. The light is reflected from the plane mirror toward the telescope s eyepiece. The light rays converge and create an image of the distant object, and, like in a refracting telescope, a convex lens in the eyepiece magnifies the image. Some telescopes collect light rays from several mirrors and them combine them into a single image (e.g. the Keck telescope in Hawaii)

30 Assignment Describe each of the following examples relating to optics. For each one, explain whether the scientific activities related to the example were group or individual. 1. Galileo: telescope 2. Newton: telescope 3. Hubble Telescope 4. Laser technologies Due date: