Bottom of a tank containing water appears to be raised. It is due to

The phenomenon where the bottom of a water tank appears raised is a direct consequence of the refraction of light. Refraction occurs when light travels from one transparent medium to another, such as from water to air, causing a change in its speed and direction [3]. When light rays from the bottom of the tank travel from the denser medium (water) to the rarer medium (air), they bend away from the normal at the interface [1]. This bending causes the light rays to appear as if they are originating from a point higher than their actual position, creating an optical illusion of a shallower depth [2]. This perceived distance is known as the apparent depth, which is typically less than the real depth. Similar effects include the apparent bending of a pencil partially immersed in water or the magnification of objects viewed through a glass slab [1].

Sources

1. [2] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > Activity 9.10 > p. 147
2. [3] https://sathee.iitk.ac.in/hi/ncert-books/class-10/knowledge-route-10/knowledge-route-10-physics/harshit_knowledge-route-class10-physics/
3. [1] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > 9.3 REFRACTION OF LIGHT > p. 145