Light — Important Questions

Straight line

Light always travels in a straight line path in a uniform medium. This property is known as the rectilinear propagation of light. Evidence for this includes: light entering a dark room through a narrow slit appears as a straight beam, and in the candle-straw activity, bending the straw prevents the flame from being seen. Shadows are also formed because light cannot bend around objects — it travels straight.

3 × 10⁸ m/s

Light travels at a maximum speed of 3 × 10⁸ metres per second (300,000,000 m/s) in air or vacuum. This is the fastest speed in the universe. Light travels slightly slower in denser media — for example, 2.25 × 10⁸ m/s in water and 2 × 10⁸ m/s in glass. The options 3 × 10⁶ m/s and 3 × 10⁴ m/s are too small, and 3 × 10¹⁰ m/s is too large.

Inverted and real

In a pinhole camera, light from the top of the object travels in a straight line through the pinhole and hits the bottom of the screen, while light from the bottom of the object hits the top of the screen. This crossing of rays makes the image upside down (inverted). Since the image is formed on an actual screen (tracing paper), it is a real image. It is also generally smaller than the object (when the object is far away) and is temporary.

The image becomes larger

The size of the image in a pinhole camera depends on the distance of the screen from the pinhole. The formula is: Size of image / Size of object = Distance of screen from pinhole / Distance of object from pinhole. So, when the screen moves farther from the pinhole, the numerator (screen distance) increases, making the ratio larger. Therefore, the image size increases. The opposite happens when the object is moved farther — the image becomes smaller.