Chapter 11 — Light: Shadows and Reflections

Chapter 11, titled 'Light: Shadows and Reflections', covers sources of light, how light travels in a straight line, how shadows are formed when opaque objects block light, and how reflection by plane mirrors works. It also explains the pinhole camera and how to make a periscope and a kaleidoscope.

Objects that emit their own light are called luminous objects — for example, the Sun, stars, lightning, natural fire, and certain animals such as fireflies. Objects that do not emit their own light are called non-luminous objects. The Moon is a non-luminous object; it only reflects the light emitted by the Sun.

Two activities demonstrate this. In the matchbox-hole activity, three matchboxes are arranged so their holes are in a straight line; a bright spot appears on the screen only when all holes are aligned. If one box is shifted, the spot disappears. In the bent-pipe activity, a candle flame can be seen through a straight pipe but not through a bent pipe — confirming that light travels in a straight line.

Light passes almost completely through transparent materials. Light passes only partially through translucent materials. Light does not pass through opaque materials at all. These differences determine whether an object blocks, partially blocks, or allows light to pass through.

Three things are needed to observe a shadow: a source of light, an opaque object, and a screen. When the opaque object blocks the light, a dark patch — the shadow — forms on the screen where light does not reach. Walls, floors, or any surface can act as the screen in daily life.

No. Changing the colour of an opaque object does not change the colour of its shadow. The shadow is simply the dark region where light does not reach, so its colour is always dark regardless of the colour of the object blocking the light.

The image formed by a plane mirror is the same size as the object, erect (upright), laterally inverted (left and right are swapped), and cannot be obtained on a screen. The image appears as far behind the mirror as the object is in front of the mirror.

Lateral inversion is the perceived left-right reversal that occurs in images formed by a plane mirror — your left appears as right in the image and your right appears as left. 'AMBULANCE' is written in mirror script (reversed) on the front of an ambulance so that when a driver ahead sees it in their rear-view mirror, it reads correctly as 'AMBULANCE', helping them recognise the emergency vehicle quickly.

A pinhole camera is a device in which light rays from an object pass through a tiny hole (a pinhole) and form an image on a screen. The image formed is inverted — upside down. The images show the colours of the objects. This is different from a plane mirror, where the image is erect but laterally inverted.

A periscope is made by placing two plane mirrors in a Z-shaped box. Reflection from the two mirrors allows a person to see objects that are not directly visible. Periscopes are used in submarines, tanks, and by soldiers to see outside their bunkers without exposing themselves.

A kaleidoscope uses three rectangular plane mirror strips joined together in a triangular arrangement. Because there are three mirrors, light undergoes multiple reflections (reflections of reflections), creating many interesting patterns with coloured beads or bangle pieces placed at one end. The pattern changes every time the kaleidoscope is turned, and designers and artists use kaleidoscopes to get ideas for new patterns.

Yes, the NCERT Class 7 Science Chapter 11 PDF is free to read and download on cbseprepmaster.com. No sign-up or account is required.