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Optical Bench- Convex Lenses KS4 Suitable for AQA P3
On a copy of the diagram below 1. Complete the path of the ray of light. (Remember to draw the normal lines.) Air Glass 2. State the name of the effect.
Light Remember light will slow down as it travels from air to glass because glass is more optically dense. Air Glass The light bends towards the normal on the way in and away from the normal on the way out. This is known as REFRACTION
Lenses Simple lenses are curved pieces of glass or plastic. They come in two basic forms: Convex A convex lens is thicker in the middle than at the edges. Concave A concave lens is thicker at the edges than in the middle.
Refraction Using your knowledge of refraction, complete the three rays of light incident on a convex lens. (It is important here to accurately draw your normal lines.)
Refraction As before, light entering the glass bends towards the normal and light leaving the glass bends away from the normal. This causes the rays of light to be brought together. For this reason convex lenses are often called CONVERGING lenses.
Drawing lenses While light passing through a lens is refracted upon entering and leaving the glass it is often easier to draw the refraction occurring at the centre of the lens.
Focal length The point at which the parallel rays of light cross is called the focal point or focus. The distance from the centre of the lens to the focal point is called the focal length. Focal point Add these labels to your diagram Focal Length
Optical Bench An optical bench allows lenses to be easily aligned and measurements taken. Set up your optical bench with the rule pointing at a distant object. Arrange the screen and a lens holder on the rule so that light from the distant object can pass through the lens and strike the screen. Put the one of the lenses in the holder and place it close to the screen. Slowly move the lens away from the screen until a sharp image is formed on the screen.
Explanation Light travelling parallel to the principal axis will be refracted through the focal point. Light that passes through the centre of the lens will be undeviated. We can then determine the position of an image by constructing a ray diagram. Object Image
Distant Objects The further the object is from the lens the closer the image gets to the focal length of the lens. Object Image
Determining focal length If you have focused a distant object onto your screen the distance between the centre of the lens and the screen will be an estimation of the focal length. Using one of the lenses from your set: Using a rule estimate the thickness of the lens at its widest point. Place the lens in a holder. Slowly move the lens away from the screen until a sharp image is formed. The distance from the lens to the screen is an approximation of the focal length of the lens. Record your results in the table provided. Repeat for the remaining lenses.
Results Thickness of lens (mm) Estimated focal length f (mm) 15 60 9 110 5 200 Look at your results and suggest a relationship between lens thickness and focal length.
Power For convex lenses of the same material the fatter the lens, the shorter the focal length. This means that a fat lens brings rays of light to a focus in a shorter distance. We say that the shorter the focal length the more powerful the lens is. Which of your lenses is the most powerful?
Optical Bench Set up your optical bench with the object and screen at opposite ends of the metre rule and one lens holder between them. Set up a bench lamp to shine through the object towards the screen. Put the 100mm lens in the holder and move the holder until a sharp image of the object is formed on the screen. What do you notice about the size of the image compared to the size of the object?
Magnification The image formed on the screen should be bigger or smaller than the size of the object. We say that it has been magnified. To calculate the magnification we use the relationship: magnification = image size object size Calculate the magnification for the lens at this position by measuring the height of the object and the height of the image. Record your results in the table.
Results Image size (mm) Object size (mm) Magnification 63 9 7 Notice that magnification has no units as it is a ratio of two lengths. A magnification of 7 indicates that the image is 7 times bigger than the object.
Magnification We can also determine the magnification using the distances between the components. Use the metre rule to determine the distance between the object and the centre of the lens. This is the object distance (d o ) Measure the distance between the lens and the screen. This is the image distance (d i ) Record these results in the table provided and calculate magnification using: magnification = d i d o
Results using image size: Results using distances: Results Image size (mm) Object size (mm) Magnification 63 9 7 Image Distance (mm) Object Distance (mm) Magnification 862 125 6.9 Comment on the quality of your results.
Collecting data Draw a table on a separate piece of paper with the following column headings. Image Distance (mm) Object Distance (mm) Magnification d i d 0 Image size (mm) Magnification image size object size Move the screen a to about 50cm from the object and focus the image again. Determine magnification in both ways and record your results in the table. You should be able to find two lens positions that create a focused image for each screen position. Increase the object / screen distance and repeat.
Fall Back Data Object Size: 9mm Image Distance (mm) Object Distance (mm) Magnification Image size (mm) 875 125 63 205 795 2 345 155 20 163 337 4 462 138 30 150 450 3 621 129 44 133 617 2 Magnification
Fall Back Data Object Size: 9mm Image Distance (mm) Object Distance (mm) Magnification Image size (mm) Magnification 875 125 7.00 63 7.00 205 795 0.26 2 0.22 345 155 2.23 20 2.22 163 337 0.48 4 0.44 462 138 3.35 30 3.33 150 450 0.33 3 0.33 621 129 4.81 44 4.89 133 617 0.22 2 0.22
Summary Questions Draw 1 thin and 1 fat convex lens. State what is meant by focal length. Indicate the lens that has the shortest focal length. An object with a height of 10cm is projected through a convex lens and produces an image that is 30cm in height. Calculate the magnification of the lens.
Solutions This lens has the shortest focal length. Thin Lens Fat Lens The focal length of a lens is the distance from the centre of the lens to the focal point. The magnification of the lens is 3.