Light ppt
- 2. Activity15.2 Place a lighted candle in front of a plane mirror. Try to see the flame of the candle in the mirror. It appears as if a similar candle is placed behind the mirror. The candle, which appears behind the mirror, is the image of the candle formed by the mirror The candle itself is the object. Now move the candle to different positions in front of the mirror. Observe the image in each case.
- 3. Was the image upright in each case? Did the flame appear on top of the candle as in the object? Such an image is called erect. An image formed by a plane mirror is erect and of the same size as the object.
- 5. Take a chess board. If a chess board is not available, draw on a chart paper 64 (8 8) squares of equal size. Draw a thick line in the middle of the paper. Fix a plane mirror vertically on this line. Place any small object, such as a pencil sharpner, at the boundary of the third square counting from the mirror. Note the position of the image. Now shift the object to the boundary of the fourth square. Again note the position of the image. Did you find any relation between the distance of the image from the mirror and that of the object in front of it? You will find that the image is at the same distance behind the mirror as the object is in front of it. Now verify this by placing the object anywhere on the chart paper.
- 7. Activity 15.4 Stand in front of a plane mirror and look at your image. Raise your left hand. Which hand does your image raise .Now touch your right ear. Which ear does your hand touch in your image? Observe carefully. You will find that in the mirror the ërightí appears ëleftí and the ëleftí appears ërightí. Note that only sides are interchanged; the image does not appear upside down. Now write down your name on a piece of paper and hold it in front of a plane mirror. How does it appear in mirror
- 8. Activity 15.6 Take a concave mirror. Hold it facing the sun. Try to get the light reflected by the mirror on a sheet of paper. Adjust the distance of the paper until you get a sharp bright spot on it .Hold the mirror and the sheet of paper steady for a few minutes. Does the paper start burning? This bright spot is, in fact, the image of the sun. Notice that this image is formed on a screen. An image formed on a screen is called a real image. Recollect that in Activity 15.2 the image formed by a plane mirror could not be obtained on a screen. Such an image is called a virtual image. Now let us try to obtain on the screen the image of a candle flame formed by a concave mirror.
- 9. Activity 15.7 Fix a concave mirror on a stand (any arrangement to keep the mirror steady would do) and place it on a table .Paste a piece of white paper on a cardboard sheet (say about15cm 10cm). This will act as a screen. Keep a lighted candle on the table at a distance of about 50 cm from the mirror. Try to obtain the image of the flame on the screen. For this, move the screen till a sharp image of the flame is obtained. Make sure that, the screen does not .Real images formed by a concave mirror obstruct the light from the candle falling on the mirror. Is this image real or virtual? Is it of the same size as the flame? Now move the candle towards the mirror and place it at different distances from it. In each case try to obtain the image on the screen. Is it possible to obtain the image on the screen when the candle is too close to the mirror .We see that the image formed by a concave mirror can be smaller or larger in size than the object. The image may also be real or virtual. Concave mirrors are used for many purposes. You might have seen doctors using concave mirrors for examining eyes, ears, nose and throat. Concave mirrors are also used by dentists to see an enlarged image of the teeth .The reflectors of torches, headlights of cars and scooters are concave in shape.
- 11. Can you now recognise the mirrors used as side mirrors in scooters? These are convex mirrors. Convex mirrors can form images of objects spread over a large area. So, these help the drivers to see the traffic behind them.
- 12. Image formed by lense You might have seen a magnifying glass. It is used to read very small print .You might have also used it to observe the body parts of a cockroach or an earthworm. The magnifying glass is actually a type of a lens. Lenses are widely used in spectacles, telescopes and microscopes. Try to add a few more uses of lenses to this list. Get some lenses. Touch and feel them. Can you find some difference just by touching? Those lenses which feel thicker in the middle than at the edges are convex lenses Those which feel thinner in the middle than at the edges are concave lenses. Notice that the lenses are transparent and light can pass through them.
- 13. Activity 15.9 Take a convex lens or magnifying glass. Put it in the path of sunrays. Place a sheet of paper as shown (Fig. 15.23). Adjust the distance between the lens and the paper till you get a bright spot on the paper. Hold the lens and the paper in this position for a few minutes. Does the paper begin to burn? Now replace the convex lens with a concave lens. Do you see a bright spot on the paper this time, too? Why are you not getting a bright spot this time? We have seen in the case of mirrors that for different positions of the object the nature and size of the image change. Is it true for lenses also?
- 14. Sunlight- White or coloured Have you ever seen a rainbow in the sky? You might have noticed that it appears usually after the rain when the sun is low in the sky. The rainbow is seen as a large arc in the sky with many colours. How many colours are present in a rainbow? When observed carefully, there are seven colours in a rainbow, though it may not be easy to distinguish all of them. These are ó red, orange, yellow, green, blue, indigo and violet.
- 16. Activity 15.11 Take a glass prism. Allow a narrow beam of sunlight through a small hole in the window of a dark room to fall on one face of the prism. Let the light coming out of the other face of the prism fall ona white sheet of paper or on a white wall. What do you observe? Do you see colours similar to those in a rainbow. This shows that the sunlight consists of seven colours. The sunlight is said to be white light. This means that the white light consists of seven colors. Try to identify these colours and write their names in your notebook. Can we mix these colours to get white light?
- 18. Activity 15.12 Take a circular cardboard disc of about 10 cm diameter. Divide this disc into seven segments. Paint the seven rainbow colours on these segments as shown in .You can also paste, coloured papers on these segments. Make a small hole at the centre of the disc. Fix the disc loosely on the tip of a refill of a ball pen. Ensure that the disc rotates freely. Rotate the disc in the daylight. When the disc is rotated fast, the colours get mixed together and the disc appears to be whitish .Such a disc is popularly known as Newton's disc.