Which of the following properties of light can be used to explain the phenomenon of shadow formation ?
A. Light travels in a straight line.
B. Light travels at a high speed.
C. Light is composed of seven colours.
D. Light does not pass through opaque objects.

The correct answer is D and A.

Shadow formation occurs when an obstacle blocks light rays. Let us analyze the key properties of light involved in this phenomenon:

1. Light travels in a straight line (Property A): This property, known as the rectilinear propagation of light, ensures that light rays do not bend around objects to illuminate the space directly behind them. Because the light travels straight, a dark region or shadow is formed behind the object.

2. Light does not pass through opaque objects (Property D): An opaque object blocks light from passing through it completely, casting a shadow on the surface behind it.

Speed of light (Property B) and the dispersion of light into seven colors (Property C) are not the primary properties that explain why shadows are formed.

Therefore, properties A and D combined explain the phenomenon of shadow formation, which corresponds to the option D and A.