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A red indicator LED typically drops about 2V while forward biased. That means that there will be 2V less across whatever is in series that limits the current (resistor/current source). 2 red LEDs that are in series will drop about 4V.
1 volt across 100 ohms will result in 10 milliamps of current 1V/100Ω = 0.01A. 10mA is usually a good amount of current for lighting a red LED. If you have 2 red LEDs in series with the 100Ω resistor and a 5V supply, then that 10mA will flow though all 3 components (and the power supply).
Why might one want to light series LEDs instead of just one LED?
With a single LED and 5V, you would need a 300Ω resistor for 3V/300Ω = 0.01A (10mA) through the resistor and LED. The power that could have lit a second LED, is instead consumed by the resistor as additional waste heat.
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