Q1: Heat waves

ANS:D - both (b) and (c).

The correct option is: both (b) and (c). Explanation:

1. Heat waves cannot pass through vacuum: This statement is generally true. Heat waves, also known as infrared radiation, can travel through a vacuum because they are electromagnetic waves and do not require a medium for propagation. Infrared radiation is one of the ways that heat is transferred from one place to another, such as from the Sun to the Earth.
2. Heat waves travel in straight lines: This statement is also true. Heat waves propagate in straight lines from their source. They follow the laws of geometric optics, which dictate that light, including infrared radiation, travels in straight lines unless it encounters an obstacle or is refracted or reflected by a medium.
3. Heat waves can be reflected by a mirror: This statement is true. Like visible light, heat waves (infrared radiation) can be reflected by surfaces, including mirrors. The reflection of heat waves follows the same principles as the reflection of light waves.