Light is a very complex phenomenon, but in many situations its behavior can be understood with a simple model based on rays and wave fronts. A ray is a thin beam of light that travels in a straight line. A wave front is the line (not necessarily straight) or surface connecting all the light that left a source at the same time. For a source like the Sun, rays radiate out in all directions; the wave fronts are spheres centered on the Sun. If the source is a long way away, the wave fronts can be treated as parallel lines.
Rays and wave fronts can generally be used to represent light when the light is interacting with objects that are much larger than the wavelength of light, which is about 500 nm. In particular, we'll use rays and wave fronts to analyze how light interacts with mirrors and lenses.
The efficiency of the device is 30 %
Explanation:
The efficiency of a heat engine is given by:
where
W is the work done by the engine
is the heat in input to the engine
For the device in this problem, we have:
W = 120 J is the work done
is the heat in input
Substituting, we find the efficiency:
which corresponds to an efficiency of 30%.
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Answer:
<em>UP</em>
Explanation:
heat flows from higher level to lower level
( higher concentration to lower concentration )
and since temperature in above block is less than the lower block, the heat will flow from lower block to higher block .
( Up )
Answer:
a)
b) 
Explanation:
The net force on the car must produce the centripetal acceleration necessary to make this circle, which is 
. At the top of the circle, the normal force and the weight point downwards (like the centripetal force should), while at the bottom the normal force points upwards (like the centripetal force should) and the weight downwards, so we have (taking the upwards direction as positive):
Which means:
The limit for falling off would be 
, so the minimum speed would be:
Answer:
Required charge 
.
Explanation:
Given:
Diameter of the isolated plastic sphere = 25.0 cm
Magnitude of the Electric field = 1500 N/C
now
Electric field (E) is given as:
where,
k = coulomb's constant = 9 × 10⁹ N
q = required charge
r = distance of the point from the charge where electric field is being measured
The value of r at the just outside of the sphere = 
thus, according to the given data
or
or
Required charge .
Now,
the number of electrons (n) required will be
or
or
