1 Refraction of Light Activity: Disappearing coin Place an empty cup on the table and drop a penny in it. Look down into the cup so that you can see the coin. Move back away from the cup slowly until the coin just disappears from your sight. Ask your friend to slowly pour water into the cup. Does the coin come back into sight? Why does this happen? Activity: Refraction of light in glass Place your glass in the middle of a piece of plain paper and sketch its outline with a pencil. Remove the glass and draw a normal line about 1 cm from the end of the glass as illustrated below. Draw an arrow 30 from the normal. Put a piece of cardboard underneath your plain paper. Place a pin at each end of the line you just drew. Carefully place your glass on the square you just drew. Kneel down and view the two pins through the glass. While looking through the glass, move your eyes from left to right until the two pins appear to line up. Place a pin near the edge of the glass to mark where the pins line up. Look through the glass again and place a 4 th pin so it lines up with the way the other three pins appear through the glass. Remove all the pins and connect the pinhole dots with a pencil. Repeat the experiment using an angle of incidence of 45. Sr. No. Angle of incidence (i) Angle of refraction (r) 1. 30 2. 45 Questions: What direction were the straight light rays bent as they moved from the air (outside the block) into glass block?
2 What direction did the straight light rays bend as they moved from inside the glass block back into air? Rules of Refraction Why does light bend when it travels from one medium to another? Define the term refractive index.
3 What is optical density? Checkpoint Consider the diagram at the right in answering the next two questions. 1. Light must travel in medium 1 compared to medium 2. 2. Light must travel in medium 2 compared to medium 3. Arthur's method of fishing involves spearing the fish while standing on the shore. Because of the refraction of light, the observed location of the fish is different than its actual location. If Arthur is to successfully spear the fish, must he aim at, below, or above where the fish appears to be? Explain.
4 Activity: Image formation in convex lens Place an object before a convex lens for each of the following object positions shown below and obtain its image. Measure the distance of the image from the mirror. Note the location, orientation (upright or inverted), relative size (larger or smaller than object), and the type of image (real or virtual) for each condition. Draw ray diagrams for each condition and verify the results observed by you. Case 1: Object is located at infinity. Case 2: Object is located beyond the center of curvature. Case 3: Object is located at the center of curvature.
5 Case 4: Object is located between the center of curvature and focal point. Case 5: Object is located at the focal point. Case 6: Object is located at between the focal point and the lens.
6 Activity: Image formation in concave lens Place an object before a concave lens for each of the following object positions shown below and obtain its image. Measure the distance of the image from the mirror. Note the location, orientation (upright or inverted), relative size (larger or smaller than object), and the type of image (real or virtual) for each condition. Draw ray diagrams for each condition and verify the results observed by you. Case 1: Object is located at infinity. Case 2: Object is located beyond the center of curvature. Lens formula, magnification and power of a lens = distance of the image from the mirror. = distance of the object from the mirror. = focal length of the mirror. Power of lens = Diopter Net power of n lens placed in contact =
7 Checkpoint One half of a convex lens is covered with a black paper. Will this lens produce a complete image of the object? Verify your answer experimentally. Explain your observations. A spherical mirror and a thin spherical lens have each a focal length of 15 cm. The mirror and the lens are likely to be 1. both are concave 2. both are convex 3. the mirror is concave and the lens is convex 4. the mirror is convex and but the lens is concave
8 Questions and Problems 1. A concave lens of focal length 15 cm forms an image 10 cm from the lens. How far is the object placed from the lens? 2. An object 5 cm in length is held 25 cm away from a converging lens of focal length 10 cm. Draw the ray diagram and find the position, size and the nature of the image formed. 3. An object of 5 cm is placed at a distance of 20 cm in front of a convex mirror of radius of curvature 30 cm. Find the position, nature and size of the image.
9 Human Eye Key parts of the eye 1. Pupil 2. Iris 3. Lens 4. Retina 5. Optic Nerve Blind spot Hold the worksheet about 20 inches away. With your right eye, look at the dot. Slowly bring the image (or move your head) closer while looking at the dot. At a certain distance, the + will disappear from sight. Why does this happen? How do we see in 3D?
10 Rods and Cones The retina is the back part of the eye that contains the cells that respond to light. These specialized cells are called photoreceptors. There are 2 types of photoreceptors in the retina: rods and cones. The rods are most sensitive to light and dark changes, shape and movement and contain only one type of light-sensitive pigment. Rods are not good for color vision. In a dim room, however, we use mainly our rods, but we are "color blind." Rods are more numerous than cones in the periphery of the retina. Next time you want to see a dim star at night, try to look at it with your peripheral vision and use your ROD VISION to see the dim star. There are about 120 million rods in the human retina. The cones are not as sensitive to light as the rods. However, cones are most sensitive to one of three different colors (green, red or blue). Signals from the cones are sent to the brain which then translates these messages into the perception of color. Cones, however, work only in bright light. That's why you cannot see color very well in dark places. So, the cones are used for color vision and are better suited for detecting fine details. There are about 6 million cones in the human retina. Some people cannot tell some colors from others - these people are "color blind." Someone who is color blind does not have a particular type of cone in the retina or one type of cone may be weak.
11 What is 20/20 vision? Power of accommodation Near point Far point Defects of vision and their correction Myopia Image is formed in front of the retina. Far point is nearer than infinity. Suggest a method to correct myopia.
12 Hyperopia (Hypermetropia) Image is formed in behind the retina. Near point is beyond than normal near point (about 25 cm). Suggest a method to correct hyperopia. Dispersion of light Activity: Dispersion of light from a prism 1. Locate the bright sun in the sky preferably in the early morning or late afternoon where it is lower in the sky and almost horizontal rays are obtained. 2. 2 The sun is a good source of light for illustrating dispersion because the rays are parallel and each ray strikes the prism at the same angle. 3. Place the prism on the template as shown so that the light beam enters one side of the prism and exits the opposite side and refracts through the largest possible angle. 4. Place a vertical white screen about 50 cm to 100 cm behind the prism such that the sunlight beam strikes the screen after passing through the prism. You need some distance to allow the colors to spread out. 5. Slowly rotate the prism until you are able to observe a bright spectrum of light on the screen. 6. Record the colors you see from top (pointed end of prism) to bottom and look up the wavelength of each color in nanometers. Color Red Orange Yellow Green Blue Indigo Violet Wavelength in nanometers
13 Explain why the light has to be parallel to observe a colorful spectrum. Checkpoint Explain the formation of a rainbow. What conditions are necessary to see a rainbow? Why does the sky appear blue? How would it appear from space? What is the color of the sun? Why is the sun red in color when there is sunrise or sunset?