Summary
Light travels along straight lines and changes direction when it hits a shiny surface (reflection). Sunlight is white light made up of seven colours — red, orange, yellow, green, blue, indigo and violet.
Light travels along straight lines, shown by the inability to see a candle through a bent pipe. A plane mirror forms an image that is erect, virtual, the same size as the object, and at the same distance behind the mirror as the object is in front. Left and right sides are interchanged in the image — the reason AMBULANCE is written in reverse on the vehicle. Concave mirrors can form real or virtual images depending on object distance; they are used by dentists and in torch reflectors and car headlights. Convex mirrors always form erect, virtual, smaller images and are used as rear-view mirrors in vehicles. Convex lenses converge light and act as magnifying glasses; concave lenses always form smaller virtual images. Sunlight is white light made of seven colours — red, orange, yellow, green, blue, indigo and violet — revealed by a prism and recombined using Newton's disc.
Key points & formulas
- 01Light travels along straight lines — a candle seen through a straight pipe cannot be seen through a bent pipe.
- 02A plane mirror forms an image that is erect, virtual, the same size as the object, and at the same distance behind the mirror as the object is in front of it.
- 03In a plane mirror image, left and right sides are interchanged (only sides swap — the image is not upside down); this is why AMBULANCE is written in reverse on the vehicle.
- 04A concave mirror can form a real and inverted image; when the object is placed very close, the image is virtual, erect and magnified. Concave mirrors are used by dentists and in torch and headlight reflectors.
- 05A convex mirror always forms an erect, virtual, smaller image and is used as a rear-view or side mirror in automobiles because it shows objects spread over a large area.
- 06A convex lens (thicker in the middle) converges light and is called a converging lens; when an object is very close it forms a virtual, erect, magnified image — this is how a magnifying glass works.
- 07A concave lens (thinner in the middle) diverges light and always forms a virtual, erect, smaller image.
- 08Sunlight is white light composed of seven colours — red, orange, yellow, green, blue, indigo and violet — as shown by splitting it through a glass prism; spinning Newton's disc with all seven colours makes it appear whitish.
Frequently asked questions
01How does the straight-and-bent pipe activity prove that light travels in straight lines?
When you look at a lighted candle through a straight pipe you can see the flame, but through a bent pipe you cannot. This shows that light cannot travel around a bend and so must travel along straight lines.
02What are the properties of the image formed by a plane mirror?
The image formed by a plane mirror is erect, virtual, the same size as the object, and at the same distance behind the mirror as the object is in front of it. It cannot be obtained on a screen.
03What is lateral inversion in a plane mirror?
In a plane mirror, the left side of the object appears on the right in the image, and the right side appears on the left. Only the sides are interchanged; the image does not appear upside down.
04Why is the word AMBULANCE written in reverse on the front of an ambulance?
When the driver of a vehicle ahead looks in the rear-view mirror, the reversed lettering reads correctly as AMBULANCE, alerting the driver to give way to the emergency vehicle.
05What is the difference between a real image and a virtual image?
A real image can be obtained on a screen; a virtual image cannot. For example, the image of the Sun focused by a concave mirror on a sheet of paper is a real image, while the image formed by a plane mirror is a virtual image.
06What are the uses of concave mirrors?
Concave mirrors are used by doctors and dentists to see enlarged images of eyes, ears, nose, throat and teeth. They are also used as reflectors in torches and as headlights in cars and scooters.
07What type of image does a convex mirror always form?
A convex mirror always forms an erect, virtual image that is smaller in size than the object. Because it can form images of objects spread over a large area, it is used as a side mirror and rear-view mirror in automobiles.
08How does a convex lens work as a magnifying glass?
When an object is placed very close to a convex lens, the image formed is virtual, erect and magnified. This magnified virtual image is how a convex lens functions as a magnifying glass.
09What is the difference between a convex lens and a concave lens?
A convex lens is thicker in the middle than at the edges and converges (bends inward) the light falling on it, so it is called a converging lens. A concave lens is thinner in the middle than at the edges and diverges (bends outward) light, so it is called a diverging lens.
10What type of image does a concave lens always form?
A concave lens always forms a virtual, erect image that is smaller in size than the object, regardless of where the object is placed.
11How many colours are present in sunlight and what are they?
Sunlight (white light) is composed of seven colours: red, orange, yellow, green, blue, indigo and violet. These are the same seven colours seen in a rainbow.
12What is Newton's disc and what does it demonstrate?
Newton's disc is a circular cardboard disc divided into seven segments painted with the seven rainbow colours. When the disc is rotated fast, the colours mix and the disc appears whitish, demonstrating that white light is a mixture of these seven colours.
13What happens when a concave mirror is held facing the Sun?
A concave mirror reflects sunlight and concentrates it to form a bright spot — the real image of the Sun — on a sheet of paper. If the mirror and paper are held steady, the paper can start burning due to the concentrated heat.
14What image does a concave mirror form when the object is placed very close to it?
When an object is placed very close to a concave mirror, the image formed is virtual, erect and magnified — it cannot be obtained on a screen.
15Is this NCERT chapter content free to access on cbseprepmaster.com?
Yes — cbseprepmaster.com provides free access to NCERT chapter content and practice questions with no sign-up required.
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