14. we can see objects when the light reflected from the object falls on our eyes. why can’t walls act as mirror even when the light is reflected through the walls and reach our eyes?

15. how long does it take the light from the nearest star other than the sun to reach earth?
(timss)
a. less than 1 second
b. about 1 hour
c. about 1 month
d. about 4 years

16.a refracting telescope uses a concave and a :

a. concave mirror
b. convex lens
c. both a and c
d. convex mirror

14. Mirror is made up of a very smooth surface, whereas wall is not made up of a smooth surface. Smooth surfaces reflect the incident light to more extent, where as rough surfaces absorb most of the incident light. Hence, the reflected light from walls can't form the image and wall can't act as a mirror.

15. Correct answer is d) About four years

16. Correct answer is b) Convex lens

Explanation:

15. If the light from the nearest star (Alpha Centauri) takes more than 4 years to reach us, we're seeing that star 4 years in the past.

16. Refracting telescopes use convex lenses, at the front of a tube, to gather and focus incoming light. The image formed is magnified for the viewer through an eyepiece lens at the back end of the tube.

explanation:

answer: semi-infinite solid initially at a uniform temperature of 400 ◦c is cooled at its surface by convection. the cooling fluid at a constant temperature of 200 ◦c has a convective coefficient (ha) of 250 w/m2.k. the physical properties of the solid: k = 20 w/m.k and α = 4 * 10-5  m2/s. using δx = 0.04 m and δt = 10 s. write the finite difference equations describing the transient heat transfer of the first five δx and the temperature profile after 30 s.