A person holds a concave mirror in his hand and directs its reflecting surface towards the Sun. He reflects the sunlight onto a sheet of paper and a sharp bright spot is formed. What will happen next?

The correct answer is Paper will burn producing smoke.

Key Points

• When sunlight is reflected on a sheet of paper using a concave mirror, the light rays converge at the mirror's focus, creating a concentrated beam of heat and light.
• The sharp bright spot formed on the paper is due to the focal point of the concave mirror, where energy density is extremely high.
• As the energy is focused on a small area, the paper absorbs the heat and its temperature rises rapidly.
• Once the temperature exceeds the ignition point of paper (around 233°C or 451°F), the paper will catch fire and burn, often producing smoke.
• This principle is based on the concentration of solar energy using optical devices like concave mirrors, which are commonly used in solar cooking and heating applications.

Additional Information

• Concave Mirror:
  • A concave mirror is a spherical mirror with the reflecting surface curved inward.
  • It is used to focus light rays at a single point due to its converging properties.
  • Applications include solar concentrators, headlights, and shaving mirrors.
• Focal Point:
  • The focal point is the point where parallel rays of light converge after reflection from a concave mirror.
  • The distance between the mirror and the focal point is called the focal length.
• Ignition Point of Paper:
  • The ignition point, also known as the flash point, is the temperature at which a material catches fire.
  • For paper, this is approximately 233°C (451°F).
• Solar Energy Concentration:
  • Concentrating sunlight using mirrors or lenses can generate high temperatures, useful for applications like solar cooking or power generation.
  • This technique leverages the optical properties of devices such as concave mirrors or Fresnel lenses.