I'm not familiar with RasPI, but datasheet for MCU AtMega328 says max voltage for logical LOW is 0.3 times Vcc, for 3.3V that would be 3.3 * 0.3 = 0.99V. So your "light blocked" voltage would have to be less than 0.99V.
For a logical HIGH, that's 0.6 times Vcc, so your "not blocked" voltage would have to be greater than 1.98V.
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