Activity 10.2 NCERT Class 10 Science, Light – Reflection and Refraction
Activity 10.2 asks us to focus sunlight on a piece of paper using a concave mirror.
Paper catches fire when we focus a concave mirror in sunlight.
A concave mirror forms a real image. When the object is at infinite like sunlight, the image formed is concentrated as a beam. If the mirror is of sufficient size, this concentrated beam of light can easily burn paper and other relevant materials.