Fill in the blanks:

The center of the sphere to which a spherical mirror belongs is called ______

লাল ,বেগুনি ,সবুজ ও  কমলা  বর্ণের আলোর জন্য কোনো মাধ্যমের প্রতিসরাঙ্ক যথাক্রমে u_R,u_V,u_G   ও u_O  হলে নিচের কোন সম্পর্কটি সঠিক?

The focal length of a concave mirror is $50\ cm.$ Where an object be placed so that its image is two times and inverted

A bucket of total height 60 cm is half filled with a liquid of refractive index 1.5 and half with another liquid of refractive Index 2. The apparent depth of the bucket for an observer directly above the bucket is

How is the refractive index of a material related to real and apparent depth?