What is the angle of incidence when the ray reflected back, when it passes from denser to rarer medium.

25 A concave lens of focal length 15 cm, forms an image 10 cm from the lens. How far is the object placed from the lens? Draw the ray diagram. 26 An object is placed at a distance of 10 cm from a convex mirror of focal length 15 cm. Find the position and nature of the image. 28 Draw ray diagrams to show image formation.
Object at C – concave mirror
Object beyond 2f – convex lens
Object at infinity – concave lens. Write the nature of the image in each case. 29 Light, of wavelength of 500 nm in air, enters a glass plate of refractive index 1.5. 
Find   
(i) speed    
(ii) frequency and     
(iii) wavelength of light in glass. 
Assume that the frequency of light remains the same in both media.?