Why our vision gets blurred, if we look at the spark that comes while welding?

The simple explanation to this effect is the temporary lapse in recognizing the intensity between the bright and the dull area of the seen viewed, which occur due to incomparable higher intensity of spark (light) from the welding. The brain recognizes the picture by the matric form built by the difference in light intensities over different areas and the difference in the frequencies of light or colour.
Biologists explain it, if we look at spark like spots, the higher light causes a light induced stress i.e. photo oxidation stress in the retina called, short term stress, which causes the blurred vision. But after some time the stress gets released thus restoring normality. Viewing high intensity light like sunlight and ionizing radiation for a long time can damage the retina leading to vision loss. That is why welding professionals are advised to use protective filter glasses, biologists advises. 

Why a starter is used in a tube light?

Tube light requires two things to function. Very high starting voltage, and high voltage to continue the operation . The starter is a circuit which breaks at regular intervals . When you break a circuit and start again, high voltage spark takes place. This voltage is further amplified through the choke which is a transformer . Thus at the beginning the starter gives high voltage sparks. Due to this the current begins to flow through the tube and the tube begins to glow. Once the tube starts, the required voltage to sustain is quite low, which is provided by the choke. If the starter is bad the tube does not start . If the choke is bad or if the gas is low, the tube does not sustain its light .

Why does the flame always rise upwards?

All fuels such as wood, match-stick or candle, contain carbon and hydrogen. When these substances burn, carbon combines with oxygen of the air and forms carbon dioxide. Similarly, hydrogen also combines with oxygen and forms water. Some other gases are also produced. As a result of this burning process, these substances produce heat and light. The burning gases are what the flame is made of. The gases produced during the combustion are lighter than air and so they rise up, as a result the flame also rises upwards. If you lightly blow the flame you will notice that it flickers. This is because the air you blow disturbs. In the absence of any such disturbance the flame remains steady.