Light is an essential part of our daily lives, helping us see the world around us. From mirrors to lenses, it follows fascinating principles that explain reflection, image formation, and even the colors in sunlight. Understanding these concepts not only helps us grasp how vision works but also how optical instruments like magnifying glasses and cameras function.
LIGHT TRAVELS ALONG A STRAIGHT LINE
Light travels along a straight line. This is demonstrated by an activity where one looks at a lighted candle through a straight pipe and then through a bent pipe. The candle flame is visible through the straight pipe but not through the bent pipe. This activity demonstrates that light travels along straight lines.
Here are some additional details that support this concept:
- You might see a beam of sunlight when it enters a room through a narrow opening.
- You may also see beams of light from the headlamps of scooters, cars, and trains.
- A beam of light can be seen from a torch, searchlight from a lighthouse, or from an airport tower.
REFLECTION OF LIGHT
Reflection of light occurs when light falls on a shiny surface, causing it to change direction.
Imp points about reflection of light:
- A shining stainless steel plate or a steel spoon can change the direction of light.
- The surface of water can act like a mirror and change the path of light, creating reflections of trees or buildings.
- Any polished or shiny surface can act as a mirror.
- A mirror changes the direction of light that falls on it, and this change of direction is called reflection of light.
- An activity using a torch with narrow slits and a plane mirror demonstrates how light gets reflected from the mirror. The torch is positioned so that its light strikes the plane mirror at an angle. Moving the torch slightly to either side changes the direction of the reflected light. Looking into the mirror along the direction of the reflected light, you can see the image of the slits.
- Objects are visible when light reflected from them reaches our eyes.
RIGHT OR LEFT
When viewing an image in a plane mirror, there is an interesting difference between the image and the viewer: the right side appears as the left side, and the left side appears as the right side.
Imp observations regarding the right-left reversal in mirrors:
- When you raise your left hand, your image raises its right hand.
- If you touch your right ear, your image appears to touch its left ear.
- Only the sides are interchanged; the image does not appear upside down.
- This principle is used in ambulances, where the word “AMBULANCE” is written in a reversed manner so that drivers looking in their rearview mirrors can read it correctly and give way.
PLAYING WITH SPHERICAL MIRRORS
Playing with spherical mirrors involves observing how curved, shining surfaces, like spoons or specialized mirrors, can create different images compared to plane mirrors.
Imp aspects of spherical mirrors:
- Types of Spherical Mirrors:
- Concave Mirror: If the reflecting surface of a spherical mirror is concave (the inner surface of a spoon), it is called a concave mirror.
- Convex Mirror: If the reflecting surface is convex (the outer surface of a spoon), it is a convex mirror.
- Image Characteristics:
- The image formed by a concave mirror can be smaller or larger than the object. The image may also be real or virtual. When an object is placed very close to the mirror, the image formed is virtual, erect and magnified.
- The image formed by a convex mirror is erect, virtual and smaller in size than the object. Convex mirrors can form images of objects spread over a large area.
- Uses of Spherical Mirrors:
- Concave mirrors are used by doctors for examining eyes, ears, nose, and throat, and by dentists to see an enlarged image of teeth. They are also used in reflectors of torches and headlights of cars and scooters.
- Convex mirrors are used as side mirrors in automobiles because they allow drivers to see traffic behind them.
IMAGES FORMED BY LENSES
Images formed by lenses depend on the type of lens used (convex or concave) and the distance of the object from the lens. Lenses are commonly used in spectacles, telescopes, and microscopes.
Convex Lens :
- Characteristics: Convex lenses are thicker in the middle than at the edges. They converge light rays and are also known as converging lenses.
- Image Formation: A convex lens can form a real and inverted image. When an object is placed very close to the lens, the image formed is virtual, erect, and magnified.
- Magnifying Glass: A convex lens is used as a magnifying glass to see objects magnified. When used as a magnifying glass, the image is virtual, erect and magnified.
- Real Image: A real image can be obtained on a paper screen by adjusting the distance between the lens and the screen.
Concave Lens :
- Characteristics: Concave lenses are thinner in the middle than at the edges. They diverge light and are also known as diverging lenses.
- Image Formation: The image formed by a concave lens is always virtual, erect, and smaller in size than the object.
- General Properties:
- Lenses are transparent, allowing light to pass through them.
- The nature and size of the image change with different positions of the object.
SUNLIGHT — WHITE OR COLOURED ?
Sunlight, which appears white, is actually composed of a mixture of different colors.
Imp points regarding the composition of sunlight:
- Rainbow: A rainbow is an example of sunlight separating into its constituent colors, typically observed after rain when the sun is low in the sky. A rainbow appears as a large arc with many colors.
- Seven Colors: A rainbow consists of seven colors: red, orange, yellow, green, blue, indigo, and violet.
- Soap Bubbles and CDs: Colorful patterns seen in soap bubbles and light reflected from compact discs (CDs) also suggest that sunlight is a mixture of different colors.
- Prism Experiment: Passing a narrow beam of sunlight through a glass prism splits the light into seven colors, similar to those in a rainbow. This demonstrates that white light consists of these seven colors. Sunlight is thus said to be white light.
- Mixing Colors: When the seven colors are mixed together, they form white light. This can be demonstrated using Newton’s disc, a circular disc divided into seven segments painted with the rainbow colors. When the disc is rotated rapidly in daylight, the colors mix and the disc appears whitish.
Questions and Answers
- Fill in the blanks:
- (a) An image that cannot be obtained on a screen is called virtual.
- (b) Image formed by a convex mirror is always virtual and smaller in size.
- (c) An image formed by a plane mirror is always of the same size as that of the object.
- (d) An image which can be obtained on a screen is called a real image.
- (e) An image formed by a concave lens cannot be obtained on a screen.
- Mark ‘T’ if the statement is true and ‘F’ if it is false:
- (a) We can obtain an enlarged and erect image by a convex mirror. (F)
- (b) A concave lens always form a virtual image. (T)
- (c) We can obtain a real, enlarged and inverted image by a concave mirror. (T)
- (d) A real image cannot be obtained on a screen. (F)
- (e) A concave mirror always form a real image. (F)
- Match the items given in Column I with one or more items of Column II.
- (a) A plane mirror: (v) The image is erect and of the same size as the object.
- (b) A convex mirror: (ii) Can form image of objects spread over a large area, (vi) The image is erect and smaller in size than the object.
- (c) A convex lens: (i) Used as a magnifying glass.
- (d) A concave mirror: (iii) Used by dentists to see enlarged image of teeth.
- (e) A concave lens
- State the characteristics of the image formed by a plane mirror.
- The image formed by a plane mirror is erect, virtual, and of the same size as the object. The image is at the same distance behind the mirror as the object is in front of it.
- Find out the letters of English alphabet or any other language known to you in which the image formed in a plane mirror appears exactly like the letter itself. Discuss your findings.
- The source does not contain the answer to this question.
- What is a virtual image? Give one situation where a virtual image is formed.
- A virtual image is an image which cannot be obtained on a screen. An example of a virtual image being formed is the image formed by a plane mirror.
- State two differences between a convex and a concave lens.
- Convex lenses are thicker in the middle than at the edges, while concave lenses are thinner in the middle than at the edges. A convex lens can form a real and inverted image. When an object is placed very close to the lens, the image formed is virtual, erect, and magnified, while the image formed by a concave lens is always virtual, erect, and smaller in size than the object.
- Give one use each of a concave and a convex mirror.
- Concave mirrors are used by dentists to see an enlarged image of teeth. Convex mirrors are used as side mirrors in automobiles.
- Which type of mirror can form a real image?
- A concave mirror can form a real image.
- Which type of lens forms always a virtual image?
- A concave lens always forms a virtual image.
- A virtual image larger than the object can be produced by a
- (ii) concave mirror
- David is observing his image in a plane mirror. The distance between the mirror and his image is 4 m. If he moves 1 m towards the mirror, then the distance between David and his image will be
- (iii) 6 m
- The rear view mirror of a car is a plane mirror. A driver is reversing his car at a speed of 2 m/s. The driver sees in his rear view mirror the image of a truck parked behind his car. The speed at which the image of the truck appears to approach the driver will be
- (iii) 4 m/s
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