A piece of iron is heated in a flame. If at first it
becomes dull red, then becomes reddish yellow and
finally turns to white hot. The correct explanation
for the above observation is possible by using
[NEET 2013]
(a) Stefan's law
(b) Wien's displacement law
(c) Kirchoff's law
(d) Newton's law of cooling
[ans with explanation!]
Answers
Wien's law of displacement deals with some daily experiences:
A piece of metal heated by a blowing torch becomes "red hot" before its longest visible wavelengths appear red, then becomes more orange-red as the temperature rises, and is described as "white hot" at very high temperatures. shorter and shorter wavelengths dominate the black body emission spectrum. Before reaching the hotter redder temperature, thermal dissipation was mainly at longer infrared wavelengths, which are not visible; However, this radiation can be felt as it warms the person's nearby skin.
Changes in the color of an incandescent bulb (producing light through thermal radiation) can easily be observed as the temperature of its filament is changed by a dimmer. As the light dims and the filament temperature decreases, the color distribution shifts towards longer wavelengths, and the light appears both redder and dimmer.
A wood fire at 1500 K emits peak radiation at about 2000 nm. 98% of its radiation is only a tiny fraction at wavelengths longer than 1000 nm and visible wavelengths (390-700 nm). As a result, a campfire can keep a person warm but is a poor source of visible light.
The effective temperature of the sun is 5778 K. Using Wien's law, the green part of the spectrum has a peak emission per nanometer (wavelength) at a wavelength of about 500 nm, close to the peak sensitivity of the human eye. On the other hand, in terms of power per unit optical frequency, the Sun's highest emission is at 343 THz or at 883 nm wavelength in the near infrared.
ANSWER: B
Answer:
Imagine Mrs. Pumphrey came to know how Tricki was actually treated. Write a brief note to the narrator on her behalf.
You may begin this way:
Dear Mr. Herriot
I recently met Mrs. Mallard when she brought her Pixie to visit dear Tricki. I spoke to her
about the great triumph of surgery that had brought him back to me. Imagine my surprise
when she told me how mistaken I wa
rkhne lga hu lol